SAN BERNARDINO, Calif. — With a young wife, infant daughter and government job, Syed Farook appeared to have arrived at a sweet-spot in life. Friends knew the 28-year-old by his quick smile, his devotion to his Muslim religion and earnest talk about cars he would restore.

They didn’t know the man authorities said was busy with his wife, Tashfeen Malik, building homemade bombs and stockpiling thousands of rounds of ammunition for a commando-style assault Wednesday on a holiday party of his co-workers that killed 14 and injured 21.

“This was a person who was successful, who had a good job, a good income, a wife and a family. What was he missing in his life?” asked Nizaam Ali, who worshipped with Farook at a mosque in San Bernardino — the city east of Los Angeles where Farook killed and died.

As authorities identified the deceased and details about Farook’s life began to take shape, the question of what motivated the slaughter remained unanswered.

The FBI was investigating the shootings as a potential act of terrorism but reached no firm conclusions Thursday, said a U.S. official briefed on the probe. Separately, a U.S. intelligence official said Farook had been in contact with known Islamic extremists on social media. Both officials spoke on condition of anonymity because they were not authorized to discuss the investigation publicly.