Workers from Henkels & McCoy, a Blue Bell, Pa.-based firm, were drilling to address an unspecified electrical problem when they apparently hit the line, officials said.
They were unsure or circumspect in their responses to many of the other questions.
In the event of a gas leak, PSE&G officials said, a contractor is supposed to call 911 before calling PSE&G. It was unclear if that happened.
A PSE&G crew arrived at the scene about an hour before the explosion. "What happened next is part of the investigation," PSE&G spokeswoman Karen Johnson said in an e-mail Wednesday.
No one was evacuated during that time, said Lt. Ronald Lunetta of the Ewing police. Asked about when crews should evacuate an area, PSE&G spokesman Michael Gaffney said, "It depends." Gaffney said he could not comment on whether the Ewing complex should have been evacuated.
Ewing Mayor Bert Steinmann also hesitated to say whether protocol was followed by the contractor and PSE&G.
"I'm not certain at this point whether it was met or not met," he said. "That's part of our investigation.
"I don't know what went wrong," he added.
A number of agencies, including federal safety officials, are investigating.
While Henkels & McCoy's role in the blast is yet to be determined, records from the federal Occupational Safety and Health Administration show the firm has been cited for a number of violations and assessed fines over the years. Some are still being disputed by the company.
OSHA proposed a $42,000 penalty against the company last year for six "serious" safety violations at a water service repair project in Neptune. Those alleged violations included failing to have sufficient inspections, and not using necessary traffic control and warning signs.
The most sizable proposed penalty, $70,000, stems from a violation at a Bayonne project. OSHA officials said in a March statement that "workers in an 11-foot-deep trench were working without the proper protection and exposed to struck-by and crushing injuries from a potential cave-in."
OSHA said the company was cited for the same violation in 2011 as well. In both cases, the company is challenging the findings.
A company spokesman did not return a request for comment Wednesday.
"This employer is a large one and they do work nationwide - that's why they've been inspected so much," OSHA spokeswoman Joanna Hawkins said. "Many inspections did not result in citations."
Gaffney, the PSE&G spokesman, said that he was "very confident" in Henkels & McCoy, and that the agency had used them for other projects.
Of the seven PSE&G workers injured in the explosion, five had been released from the hospital by Wednesday afternoon, Gaffney said. The other two did not have life-threatening injuries.
In the Mercer County community of Titusville, Janssen Pharmaceutical Cos., a division of Johnson & Johnson, was mourning the loss of a coworker. Cerritelli was a regional office coordinator for the company, a J&J spokesman said.
Michael Yang, Janssen president, said in a statement that Cerritelli "was a valued colleague, and our thoughts and prayers are with her family and friends during this incredibly difficult time."
In her neighborhood, residents of 22 homes rendered uninhabitable were escorted back in brief forays Wednesday to get essential belongings or medicines. A total of 55 units were damaged in Tuesday's blast.
Bill Kauffman, 59, said his wife grabbed blouses and checkbooks, making a "straight line" to the closet and kitchen. Then she had to leave, not knowing when they could return.
"When you're out of your element, it's a little challenging," he said. "And we just have to take it one day at a time."
On Wednesday, the only sign of the unit that exploded was a mangled pile of scorched rubble through which a bulldozer tore. Shards of glass littered the grass near adjacent units. Most of the units in the complex were still standing, but some had boarded-up windows and roofs that were partly peeled off, one with a TV dish tipped over.
Inquirer staff writers Angelo Fichera and Jonathan Lai contributed to this article.