The explosion Monday occurred in a fifth-floor boiler room where pipes feed natural gas into a water heating system.
"We have the experts lining up to make the determination about what specifically caused it," said Cuyler McGinley, district operations manager for Hensel Phelps Construction Co.
The 1,190-room, 30-story Hilton San Diego Bayfront hotel is scheduled to open in December next to the San Diego Convention Center.
McGinley said he didn't know whether it would be ready, but the company is optimistic. A message left for Karima Zaki, the hotel's vice president for new development, was not immediately returned.
San Diego Fire-Rescue spokesman Maurice Luque said an electrical spark or boiler flames could have ignited the gas cloud.
An investigation by the state Division of Occupational Safety and Health will take two to six months, said spokeswoman Kate McGuire. She declined to speculate on the cause.
Three workers from subcontractors were in critical condition Tuesday with second- and third-degree burns and were in medically induced comas, said Kimberly Edwards, a spokeswoman for University of California, San Diego, Medical Center. Another worker who crashed a car in an adjacent parking lot after being blinded by the explosion remained hospitalized with head and neck injuries but was in good condition, said Edwards.
Another worker remained in fair condition at Scripps Mercy Hospital, said hospital spokeswoman Kristin Reinhardt.
More than 400 construction workers were at the site at the time.
Hensel Phelps, based in Greeley, Colo., rebuilt sections of the Pentagon after the Sept. 11, 2001, terrorist attacks and has worked on airports and courthouses, as well as projects for the U.S. military and Bureau of Prisons.