An explosion in San Francisco's Financial District this morning believed to have been caused by a faulty electrical transformer blew off a manhole cover, shattered windows on nearby stores and sent one woman to the hospital with critical injuries.
Authorities weren't sure what caused the explosion at 9:50 a.m. at Post and Kearny streets, but said it was probably related to one of the Pacific Gas and Electric Co. transformers located underneath that corner.
The blast buckled a sidewalk on Kearny and blew out windows in the Polo Ralph Lauren clothing store in the Crocker Galleria shopping center.
Police, initially wary that the explosion could have been caused by a bomb, evacuated hundreds of people from three nearby buildings. But investigators quickly concluded that there was no evidence of terrorism, said San Francisco Deputy Police Chief Morris Tabak. Tabak said the injured woman was a 41-year-old pedestrian who was walking along Kearny when the manhole cover blew off.
She suffered burns to the head and the neck and was taken to the Burn Center at St. Francis Memorial Hospital. Cheryl Fama, president and CEO of the hospital, said the woman was conscious when she arrived but was in critical condition.
"She was just at the wrong place" at the time of the blast, Tabak said. Tabak said a preliminary investigation indicated that the cause of the explosion was electrical.
"That's why the sidewalk buckled and imploded," he said. "It appears it was a quick, hot flash."
A bomb squad that scoured the area came up with no signs of an explosive device.
"In our response we look at every event with the added potential of terrorism when we respond to it," Tabak said. "We have a global viewpoint."
Mayor Gavin Newsom, Police Chief Heather Fong and Fire Chief Joanne Hayes-White all responded to the explosion.
Kearny Street from California Street to Market Street was closed to traffic. The incident did not affect the California Street cable car line, but a number of Municipal Railway buses were being rerouted.
Police Officer Gary Peachey said all the windows were blown out of the Polo Ralph Lauren store at 90 Post St. The explosion also sparked a fire in the awning of the store.
Paul Moreno, a spokesman for PG&E;, said there was no advance warning of the suspected malfunction, but that a system alert had gone off at the time of the explosion.
The blast did not cause any widespread power outage.
Hundreds of people crowded the street in front of the Crocker Galleria after the evacuations. Among them was Tom McGibben, who works at Thomas Weisel Partners at 120 Kearny St.
"We saw the lights flicker at 9:50 a.m," McGibben said. "We felt a pretty good shake. At first, we thought it was an earthquake.
"We saw smoke," he added. "The alarms in our building came on. ... At around the 20th floor you could smell something. At that point, we began to think it could be something worse than a quake.
"There were flashes to 9/11," McGibben said.
Tim Keenan, an employee at Charles Schwab, said he was evacuated from the 17th floor of the same building. "I saw a flash of light. I looked out of the window. I saw people running and screaming," he said. "Our floor warden then asked people to begin to evacuate. To me, it sounded like a big boom of thunder."
It wasn't the only terror-related scare in the city this morning. Authorities at the Federal Building briefly kept people away from the Larkin Street entrance after finding an unattended backpack nearby. The backpack turned out to be filled with clothes and CDs.
Staff writer Charlie Goodyear contributed to this report.
E-mail the writers at jguthrie@sfchronicle.com, jvanderbeken@sfchronicle.com and sherel@sfchronicle.com.
An explosion from an underground utility chamber in downtown San Francisco severely injured a woman and shattered a window at a Ralph Lauren clothing store shortly after 10:00 a.m. Pacific time Friday.    According to a caller to KCBS radio, a woman was engulfed in flames after the explosion triggered a fire in the store.  The fire was quickly put out.

A bomb squad is currently investigating the explosion, and the cause has yet to be determined.  One official said the explosion may have been caused by an electrical transformer. A witness described a fireball coming out of the side of the building

Pacific Gas & Electric Co. company spokesman Paul Moreno has stated that "there was no indication of a gas leak." Moreno also described the aftermath of the blast. "The explosion did displace manhole covers — two round steel ones — and it also displaced a concrete cover as well," Moreno said.

Some accounts have described a rift in the sidewalk caused by the explosion below.

Construction worker Tom Demartini, who was sitting in his truck outside the Ralph Lauren store, stated he saw the sidewalk rise up six or seven inches. "It sounded like a big poof, then there was a lot of smoke," Demartini said. "One woman looked like she was badly burned." 

The burned woman was taken to the emergency room at St. Francis Hospital.  Hospital spokeswoman Linda Gillespie had no immediate comment on her condition. Her name was not released. 

Workers in a nearby office building described how the blast shook their buildings and that they thought it was an earthquake. According to one woman, the elevator in her building stopped. 

Scores of police have evacuated buildings several blocks around the explosion.  Kearny street between Sutter and Market has been closed to both vehicles and pedestrians.