BOSTON - Law enforcement officers investigating the Boston marathon bombings searched the suburban apartment of a man being questioned in connection with the attack late Monday into early Tuesday, WBZ-TV reports.

Two bombs exploded in the crowded streets near the marathon finish line, killing at least three people and injuring more than 140 in a bloody scene of shattered glass and severed limbs that raised alarms that terrorists might have struck again in the U.S.

CBS News senior correspondent John Miller reports the man is a Saudi national currently under guard at an area hospital. He was seen running from the explosion, and a civilian chased him down and tackled him. He was turned over to Boston police and was being interviewed by the FBI. He was being cooperative and denying any involvement and it was too soon to call him a suspect.

"He was at the scene along with many other people when the blast happened," Miller said Tuesday on "CBS This Morning." "As everybody is standing in shock, three Boston PD detectives see this guy moving quickly out of the crowd. As they're watching him, he seems to be moving very deliberately away, which could be a very natural thing after a bombing. They stop him because he's covered with blood ... they think he may be injured ... they engage him, they start asking questions ... there are things about his responses that made them uncomfortable, so they arranged to get him to the hospital.

"He's somebody that they're really interested in."

Massachusetts State Police confirm to The Associated Press that a search warrant related to the probe was served Monday night in Revere, Mass., but provided no further details.

Some investigators were seen leaving the apartment building early Tuesday carrying brown paper bags, plastic trash bags and a duffel bag.

CBS station WBZ reports the search lasted nine hours.

Two men were questioned by federal agents in the lobby overnight and handed over their passports before they were allowed to go upstairs, WBZ reports.

Miller says the man was being treated for burns on his hands, and authorities suggest he may be in the U.S. on a student visa.

The two detonated devices will also provide critical information to investigators, who will "pick up every scrap, every fragment" to conduct chemical analysis. According to Miller, the early analysis indicates that the bombs were "not very sophisticated" and likely made with "something very simple like smokeless black powder, a low-order explosive ... but enough to do the job in a big crowd." He explained that it is too early to ascertain the bomber's so-called "signature," which could help authorities narrow down possible suspects.

WBZ reports one of the dead was an eight-year-old boy, identified by the Boston Globe as Martin Richard. Richard's sister reportedly lost a leg and his mother was seriously injured.

A White House official speaking on condition of anonymity because the investigation was still unfolding said Monday's attack was being treated as an act of terrorism.

President Obama, speaking earlier from the White House, pointedly avoided using the words "terror" or "terrorism," saying officials "still do not know who did this or why."

"We will find out who did this. We'll find out why they did this," Mr. Obama said in his brief statement. "Any responsible individuals, any responsible groups, will feel the full weight of justice."

Mr. Obama received updates overnight from his Assistant for Homeland Security and Counter-terrorism, Lisa Monaco, and was to get further briefings from senior administration officials later in the morning, reports CBS News correspondent Major Garrett.

As the FBI took charge of the investigation, authorities shed no light on a motive or who may have carried out the bombings. Officials in Washington said there was no immediate claim of responsibility.

The Pakistani Taliban have denied any role in the bombings, according to the AP.

Miller reported earlier that authorities are also reviewing surveillance video that shows a man from behind carrying two backpacks near the site of the explosions. Authorities are not sure whether the subject in the video is linked to the blasts.

Law enforcement sources have said there were no other unexploded devices recovered, but Mass. Rep. Bill Keating, a member of the House Committee on Homeland Security, told WBZ two other devices were found in the vicinity. He provided no other details.

The fiery twin blasts took place almost simultaneously and about 100 yards apart, knocking spectators and at least one runner off their feet, shattering windows and sending dense plumes of smoke rising over the street and through the fluttering national flags lining the course.

WBZ obtained video of one of the blasts from a runner's perspective.

When the second bomb went off, the spectators' cheers turned to screams. As sirens blared, emergency workers and National Guardsmen assigned to the race for crowd control began climbing over and tearing down temporary fences to get to the blast site.

A pool of blood formed, and huge shards were missing from window panes as high as three stories.

"They just started bringing people in with no limbs," said runner Tim Davey, of Virginia. He said he and his wife, Lisa, tried to keep their children's eyes shielded from the gruesome scene inside a medical tent that had been set up to care for fatigued runners, but "they saw a lot."

Hospitals reported at least 144 injured, at least 17 of them critically. The injuries ranged from cuts and bruises to amputations. Many victims suffered lower leg injuries and shrapnel wounds. Some suffered ruptured eardrums.

At Massachusetts General Hospital, Alasdair Conn, chief of emergency services, said, "This is something I've never seen in my 25 years here ... this amount of carnage in the civilian population. This is what we expect from war."

Demi Clark, a runner from North Carolina who said she was the crossing finish line as the first blast went off, told CBSNews.com "blood was everywhere instantly."

Boston Marathon map updated with JFK Library location CBSNews/Stamen

"Nobody knew what to do - after the second one went off we were like, 'the city's under attack,'" Clark said.

James Minicucci, who was arriving in Boston by car to meet friends at the finish line when the explosions happened, told CBSNews.com the scene was "chaotic."

"Some guy told us it was really bad, that several people lost their legs, there were amputations and not to go through to finish line area," Minicucci said.

Some 23,000 runners took part in the race, which attracts more than 500,000 spectators and winds up in the heart of central Boston, near the landmark Prudential Center and the Boston Public Library. It is held on Patriots Day, a Massachusetts state holiday that commemorates the first battles of the American Revolution in 1775.

Boston Police Commissioner Edward Davis asked people to stay indoors or go back to their hotel rooms and avoid crowds as bomb squads methodically checked parcels and bags left along the race route. He said investigators didn't know precisely where the bombs were planted or whether they were hidden in mailboxes or trash cans.

He said authorities had received "no specific intelligence that anything was going to happen" at the race.

The president was briefed on the incident Monday by several senior administration officials, including FBI Director Robert Mueller and Homeland Security Secretary Janet Napolitano. He also spoke with Massachusetts Gov. Deval Patrick and Boston Mayor Tom Menino and pledged to provide whatever federal support was needed.

In addition, Mr. Obama spoke with Republican and Democratic leaders on Capitol Hill, saying that, "On days like this, there are no Republicans or Democrats. We are Americans united in our concern for our fellow citizens."

The Federal Aviation Administration created a no-fly zone over the site of the explosions, and briefly ordered flights bound for Boston's Logan International Airport held on the ground at airports around the U.S.

A few miles away from the finish line and around the same time, a fire broke out at the John F. Kennedy presidential library. The police commissioner said it may have been caused by an incendiary device but didn't appear to be related to the bombings.

The first loud explosion occurred on the north side of Boylston Street, just before the photo bridge that marks the finish line. The second explosion could be heard a few seconds later.

They occurred about four hours into the race and two hours after the men's winner crossed the line. By that point, more than 17,000 of the runners had finished the race, but thousands of others were farther back along the course.

The four-hour mark is typically a highly crowded time near the finish line -- both because of the slow-but-steady recreational runners likely to be completing the race and because of all the relatives and friends clustered around to cheer them on.

Runners in the medical tent for treatment of dehydration or other race-related ills were pushed out to make room for victims of the bombing.

At the White House, the Secret Service expanded its security perimeter after the attacks, shutting down Pennsylvania Avenue and cordoning off the area with yellow police tape. Several Secret Service patrol cars blocked off entry points, although the White House was not on lockdown and tourists and other onlookers were still allowed in the park across the street.

At Congress, members of intelligence committees said they expected to be briefed on the attack on Tuesday.

A woman who was near the second bomb, Brighid Wall, 35, said that when it exploded, runners and spectators froze, unsure of what to do. Her husband threw their children to the ground, lay on top of them and another man lay on top of them and said, "Don't get up, don't get up."

She said she saw six to eight people bleeding profusely, including one man who was kneeling, dazed, with blood coming down his head. Another person was on the ground covered in blood and not moving.

"My ears are zinging. Their ears are zinging. It was so forceful. It knocked us to the ground."

Competitors and race volunteers were crying as they fled the chaos. Authorities went onto the course to carry away the injured while race stragglers were rerouted away from the smoking site.

Roupen Bastajian, a 35-year-old state police officer from the neighboring state of Rhode Island, had just finished the race when they put the heat blanket wrap on him and he heard the blasts.

"I started running toward the blast. And there were people all over the floor," he said. "We started grabbing tourniquets and started tying legs. A lot of people amputated. ... At least 25 to 30 people have at least one leg missing, or an ankle missing, or two legs missing."

The Boston Marathon honored the victims of the December shooting in Newtown, Conn. with a special mile marker in Monday's race.

Boston Athletic Association president Joanne Flaminio previously said there was "special significance" to the fact that the race is 26.2 miles long and 26 people died at Sandy Hook Elementary school.

Cities worldwide stepped up security following the explosions.

In Britain, police said they were reviewing security plans for Sunday's London Marathon, the next major international marathon. Thousands of people compete in the London Marathon every year, thronging the city's streets. London is also considered a top target for international terrorists.

A London Metropolitan Police spokesman confirmed Monday that police are working with marathon officials to review security plans for Sunday's event. The London race's chief executive, Nick Bitel, expressed shock and sadness about the situation in Boston, saying "it is a very sad day for athletics and for our friends in marathon running."

In New York City, police spokesman Paul Browne said that critical response teams are deployed around the city. Officials were stepping up security at hotels and other prominent locations.

Spectator Cherie Falgoust was waiting for her husband, who was running the race.

"I was expecting my husband any minute," she said. "I don't know what this building is ... it just blew. Just a big bomb, a loud boom, and then glass everywhere. Something hit my head. I don't know what it was. I just ducked."