UPDATE: The Columbia Fire Department has withdrawn a claim made on Monday night that the explosion was caused by human error. The department says the claim was "premature," and that the investigation is still ongoing as of Tuesday morning.
COLUMBIA — Four people were injured when a lab device on the third floor of MU's Schweitzer Hall at 503 S. College Ave. exploded Monday afternoon.
"Certainly this was an accidental explosion, but the lab looks like a bomb went off," said Columbia Fire Department Capt. Eric Hartman, who was on the scene.
In a news release, Hartman said the explosion was caused by an anaerobic hood — an oxygen-free chamber used for working with bacteria that can't survive in oxygen. An explosion resulted when a flammable combination of gases used for the device came into contact with an ignition source, the release said. The source of the ignition remains unknown.
A 2,000 pounds per square inch hydrogen tank did not explode, as first reported. Investigators said it remained intact.
The department also withdrew a statement issued Monday night that attributed the explosion to human error. In the Monday night release, the department said lab personnel ignored a "warning system" designed to tell researchers when too much hydrogen enters the chamber and becomes flammable.
On Tuesday morning, Columbia Battalion Chief Steven Sapp said the department's claim was "premature" and that the investigation was still ongoing. Sapp said investigators were still investigating the possibility of technical failure, and that the department might need to send the lab equipment to be evaluated by independent investigators.
The lab technician working with the anaerobic hood suffered a life-threatening impact injury to the chest in addition to burns, Hartman said. That person was transferred to University Hospital by ambulance after the explosion, as were two others with moderate injuries; the fourth person with minor injures was transferred to the hospital via a "private vehicle," Hartman said.
One person suffered difficulty breathing related to asthma, Hartman said. The four suffered various impact and shrapnel injuries in addition to burns.
As of 5:45 p.m., according to an MU News Bureau release, three of those injured had been treated and released. The fourth person remained in the hospital and was listed as being in good condition. The bureau did not specify which of four injured people remained in the hospital.
Hartman said one graduate student and two postdoctoral students were among the people injured in the explosion.
Both the fire department and the University of Missouri Police Department declined the release their names, referring reporters to the university. On Tuesday morning, MU spokeswoman Mary Banken said the university would not be releasing the names until the university had gotten permission from the victims.
Hartman said the number of injuries was "certainly reduced" because of MU being on summer break, which meant fewer people working in the facility.
The lab belongs to MU biochemistry professor Judy Wall, Basi said. Wall declined to comment on the incident or on the specific details of the experiment being conducted at the time of the explosion.
Wall's lab was used for experiments with bacteria. Hartman said he was unsure whether the 2,000 psi hydrogen tank was being used at the time of the explosion. He said the department is taking precautions against any potentially hazardous chemicals.
He said the ceiling in the lab collapsed but that the building remained structurally sound.
After the explosion, the building was evacuated, and Hartman said the fire was contained in about 10 minutes. Fire crews then began working to ventilate chemicals. The fire alarm was triggered and the building's fire sprinklers activated, extinguishing most of the fire, Hartman said in a release.
The building's central staff were allowed to re-enter the building, Hartman said at around 4:15 p.m.
Seventeen windows were blown out in the third-floor explosion. The room's number was not yet available.
Phil Leibu was working in the basement of Schweitzer Hall when he heard an explosion and saw debris falling on the ground through his window.
Jeanie Phipps, an administrative assistant in Mumford Hall, said she heard a loud boom.
"I didn't really look. I thought it was something with a truck," she said, citing the heavy roadwork on campus.
She received an e-mail alert about the explosion and went outside to see what was going on. Several small groups of people were watching the scene while the fire department was investigating, she said.
There was significant damage to the building with glass and debris falling to the ground, Hartman said. All the windows on one corner of the building were broken.
Basi said no one other than the fire department would be allowed to access the lab until further notice, and cleanup in the lab would start later Monday night.
Explosions in anaerobic chambers are not unprecedented, according to a research paper written by Mike Cox of Anaerobe Systems in San Jose, Calif.
Cox — who previously experienced two small anaerobic chamber explosions — wrote that when a chamber is transitioning to an oxygen-free state, specific combinations of oxygen and hydrogen can become flammable, making an explosion possible if something ignites the gas.
MU is expected to investigate the explosion and take steps to avoid a future incident, the fire department said in its release.
Nicholas Jain, Erica Hunt, Krista Schmidt and Emily Smoucha contributed to this report.
A pile of debris sits outside Schweitzer Hall as cleanup crews went through the building.
Workers began replacing the building's windows on Tuesday.

An explosion at the  (Mizzou) on Monday afternoon left four people injured, authorities say.

The explosion occurred in a science laboratory in Schweitzer Hall around 2:20 p.m.  (1920 UTC) Monday. The source of the explosion was first thought to have been a 2,000-pound (907.2-kilogramme)  tank, but fire officials later said that this was not the case. The cause of the incident is currently under investigation by the  Fire Department.

The Columbia Fire Department arrived at Schweitzer Hall after a report of a structure fire, but found that most of the fire had already been extinguished by the building's . The remaining flames were put out by firefighters, one of whom said it looked as if "a bomb went off in the lab".

Of the four hurt in the blast, one was a research scientist, one a graduate student, and the other two postdoctoral fellows. Three were treated for mild injuries and released from , while the fourth was in good condition after being taken to the hospital's  for life-threatening injuries. A school spokesperson said that the university was not allowed to release the names of the victims.

Authorities initially believed that a large container of hydrogen gas had exploded, but investigators later said that the tank was intact. Fire officials also retracted an earlier statement that said the incident had been a result of human error. In a Monday night news release, the fire department said that lab workers had turned on the hydrogen but did not recognize warning signs indicating a dangerously high level of hydrogen gas in the lab, so they left the gas supply running. The report said the explosion occurred after the gas reached a source of ignition. However, the department said Tuesday that the investigation into the explosion is still ongoing and that they were not certain human error was the cause.

The investigation should determine the cost of repairs for the building, as well as whether the school should implement new procedures to avoid similar incidents in the future. The lab where the explosion took place will be totally rebuilt.

Schweitzer Hall houses Mizzou's  department, which is part of the medical and agricultural programs. The building's single classroom is located in the basement and was not in use at the time. The explosion happened on a third-story lab assigned to Judy Wall, a university professor, who was in her office across from the lab during the incident and referred inquiries to the news bureau. Other labs in the building were not disturbed, and Schweitzer Hall was established to be structurally sound, although nearly twenty windows had been shattered from the blast. The building reopened Tuesday for researchers to continue working.