Grim details emerged Thursday about the massacre at a California holiday party that left 14 people dead and 21 others hurt, but investigators are still trying to answer the key question: Why?

The couple who unleashed the bloodbath Wednesday fired as many as 75 rounds at the Inland Regional Center in San Bernardino and tried to slaughter more people with a rigged pipe bomb that never detonated, authorities said Thursday.

Syed Rizwan Farook, 28, who was born in Chicago, and his wife, Tashfeen Malik, a 27-year-old naturalized by marriage U.S. citizen from Pakistan, were killed in a shootout with police a few miles away more than four hours after the rampage.

The couple — clad in black tactical gear but not bulletproof vests as originally reported — fired 76 more rifle rounds during a pursuit with police, San Bernardino Police Chief Jarrod Burguan told reporters. Officers returned about 380 rounds.

Authorities said they are still trying to determine why Farook and his wife attacked the Inland Regional Center.

But authorities told NBC News on Thursday that Farook appeared to have been radicalized. They said he had been in touch with people in the Los Angeles area who have expressed jihadist-oriented views.

Intelligence sources also said Farook appeared to have been in some form of communication with people overseas who are persons of interest to U.S. authorities.

Farook and Malik had two assault rifles and two semiautomatic handguns that were legally purchased and registered, Burguan said.

In total, the pair carried more than 1,400 .223 caliber rounds and 200 more 9mm rounds during the gunbattle. The three pipe bombs found at the social services center were stitched together on a remote-controlled car, but for whatever reason, it didn't explode, he said.

A search of a home connected with the couple turned up 12 pipe-bomb-type devices and a cache of ammunition, Burguan said — including 2,000 9mm rounds, 2,500 .233 caliber rounds and several hundred .22 Long Rifle rounds.

Police also confirmed that the couple had rented a black Ford Expedition three or four days before the carnage. The car, which was supposed to have been returned Wednesday, was used as their getaway.