LOS ANGELES - By the time the married couple who carried out the deadly San Bernardino attack came to the attention of police, it was far too late.

Syed Rizwan Farook and his wife, Tashfeen Malik, had gone undetected while planning the massacre that included amassing high-powered guns, pipe bombs, and thousands of rounds of ammunition.

The FBI's acknowledgment that the San Bernardino shooters had been radicalized Muslims for "quite some time" points to the difficulty discovering potential terrorists who keep a very low profile.

"It appears these people were very good at hiding their intentions," said David Schanzer, a Duke University public policy professor who runs a center that studies terrorism. "What this situation shows is it's not a fool-proof system. . . . A hundred percent prevention is not achievable."

The couple, who lived quietly in a two-bedroom townhouse with their 6-month-old daughter and Farook's mother, had not come to the attention of law enforcement before bursting into an annual festive meeting of Farook's county health department colleagues with guns blazing. They killed 14 people and wounded 21 last Wednesday before dying in a shootout with police about four hours later.

David Bowdich, chief of the FBI's Los Angeles office, told reporters the agency was searching for how and where radicalization occurred and who might have led them to those beliefs.

Investigators believe Malik radicalized before meeting Farook, FBI spokeswoman Laura Eimiller said Tuesday, though she did not elaborate on what led the FBI to that conclusion. Farook, 28, a restaurant inspector born in the U.S. to a Pakistani family, radicalized before Malik, 29, emigrated to the U.S. from Pakistan in July 2014 on a fiancée visa and married him the next month.

So far, the FBI has revealed little of what it's learned about the couple and their planning, except for details about the weaponry they had and that both had been taking target practice. Malik also practiced at Riverside Magnum Range, where Farook shot at targets two days before the attack, Eimiller said.

Investigators also are trying to determine a money trail for funding of the operation.

A U.S. official said Tuesday that authorities were looking into a deposit made to Farook's bank account before the shooting. The official, who had been briefed on the investigation but was not authorized to discuss it by name, would not characterize the nature of the deposit or what was suspicious about it.