Some workers were burned and others were injured by flying debris, Luque said. The building did not burn after the explosion, which Luque said appeared to be accidental.
UCSD Medical Center was treating 10 injured, said Dr. Irving Jacoby, attending emergency room physician. Three were in the burn unit, one with burns over 35 percent of his body.
"They talked about a bright light and an intense heat," said Dr. Bruce Potenza of the hospital's burn unit.
San Diego Deputy Fire-Rescue Chief Perry Peake said the blast occurred in an area of the building containing gas, electric and other utilities and that it appears to have been caused by a mechanical failure.
"We want to rule out all possibilities before making an official report, but we're pretty confident that's how it's going to come out," he said.
The blast damaged floors four through seven of the building, Mayor Jerry Sanders said.
The hotel near the San Diego Convention Center showed serious damage and debris littered a driveway beneath one corner of the structure. Facade material dangled from the structure, drapes dangled out windows, and a large equipment room with tanks and pipes was left exposed.
Firefighters searched the building to determine if there were any other victims. More than 400 construction workers were at the site at the time, Sanders said.
Matt McBride, general manager of the Tin Fish bar about 150 yards away, said he was setting up the outdoor patio bar when the explosion happened. He said he and other workers froze as they saw smoke come out of the building but no flames.
"It looked like a bomb," McBride said. "It shook the whole building like a bomb. The reverb was what scared us. Everybody was saying bomb, bomb, bomb, terrorist, terrorist, terrorist."
The site was surrounded by firefighters and utility crews, and a truck with the markings of the Bureau of Alcohol, Tobacco, Firearms and Explosives arrived.
The Padres baseball team, which plays at Petco Park across from the convention center, planned to host the St. Louis Cardinals Monday night, as scheduled, said Sandy Alderson, the team's chief executive officer.
Bruce Ragland, who runs an entry gate at the stadium, said he was looking right at the building when the explosion blew through two sides and there was a strong shock wave.
"It was just like watching a Bruce Willis movie, like you know, 'Die Hard,"' Ragland said.
The 30-story Hilton San Diego Bayfront hotel was scheduled to open in December, according to Hilton's Web site. It will have 1,190 rooms and more than 165,000 square feet of meeting space.
Karima Zaki, the vice president for new development for Hilton Hotels Corp., said she didn't know if the hotel would open on schedule. She said Hilton hadn't taken possession of the hotel from the builder, Hensel Phelps Construction Co.
A woman who answered the phone at Greeley, Colo.-based Hensel Phelps' Southern California regional headquarters referred questions to executives at the site, who were not immediately available. Hensel Phelps officials did not immediately respond to questions sent by e-mail.
---
Associated Press writer Elliot Spagat contributed to this report.
