Rescue workers at the scene of shooting

The married couple who police say killed 14 people at a social services centre  in San Bernardino, southern California,  had built more than a dozen pipe bombs and stockpiled thousands of rounds of ammunition, officials said Thursday, and they fired as many as 150 bullets at victims and police officers in a rampage that shattered a quiet day and ended in their own deaths.

The F.B.I. is treating the shooting as a potential terrorist act, though they are far from concluding that it was, two law enforcement officials said Thursday. The suspects’ extensive arsenal, their recent Middle East travels and evidence that one had been in touch with people with Islamist extremist views, both in the United States and abroad, all contributed to the decision to refocus the investigation. But officials emphasized that they were unclear what set off the attack, and said they were not ready to call it terrorism.

Syed Rizwan Farook: one of the gunmen suspected to have links with extremists

The suspects, Syed Rizwan Farook, 28, and Tashfeen Malik, 27, are believed to have opened fire inside the Inland Regional Centre  on Wednesday.

In addition to the 14 people killed, the authorities now say 21 were injured, four more than originally said. The attack was the nation’s deadliest mass shooting since the assault on an elementary school in Newtown, Conn., nearly three years ago.

“Clearly they were equipped and they could have done another attack,” police  Chief Jarrod  Burguan said at a news conference on Thursday. “We intercepted them before that happened, obviously.”

Law enforcement officials said the F.B.I. had uncovered evidence that Mr. Farook was in contact over several years with extremists domestically and abroad, including at least one person in the United States who was investigated for suspected terrorism by federal authorities in recent years, but had not been charged. The officials spoke on the condition of anonymity because they were not authorized to discuss the matter publicly.

Read more in New York Times: FBI counterterrorism officials are overseeing the investigation