Fire officials said Wednesday that natural gas was the focus of their investigation of the explosion that killed two people and leveled three units of a condominium complex early Tuesday.

West Haven Fire Chief William Johnson said investigators were still looking at all possible causes, but that a gas explosion was “the prime suspect” in the 3:30 a.m. blast at Orchard Hill condominiums.

The explosion probably occurred in Unit 7 of the complex, Johnson said. That condominium was the home of Conrad Palmer, 70, who died shortly after the explosion.

Where the gas came from and why it ignited are still unknown, Johnson said.

Johnson said fire and building officials were trying to get occupants of 18 of the complex’s 46 units back into their homes Wednesday. At the latest, they should return today, he said.Occupants of the remaining units, he said, won’t be returning for a while. Some of the nine buildings in the complex may have to be demolished, he said.

Beryl Lyons, spokeswoman for the state Department of Public Utility Control, said officials in her office are looking at records to determine whether there was any work done in the area that might have disrupted a gas line or caused the explosion in any other way. Initial inquiries have produced no records of any recent projects in the area, she said.

She added that a full investigation by her department won’t be possible until fire and police officials allow state public utility officials to inspect the site.

Officials at Southern Connecticut Gas Co., which provides fuel for the complex, did not return phone calls Wednesday.

Myriam Clarkson of Renaissance Collaborative Inc. of New Haven, the property manager for the condominium complex, said Wednesday that she did not know of any complaints of gas odors or any other problems that could have foreshadowed the blast.

Viviana Tindell, 47, was also killed in the blast and her husband, Foster, was injured. He was released Wednesday from Yale-New Haven Hospital. The Tindells lived in Unit 6, next to the unit where investigators are searching

for the source of the explosion.

David and Deborah Papin lived in Unit 8, which was also destroyed. They and their two young children managed to escape uninjured.