SAN BERNARDINO • The husband and wife behind the shooting in California that left 14 people dead were radicalised for "quite some time" and both went for target practice before the massacre, the authorities have said.

The update from the Federal Bureau of Investigation (FBI) on the terror probe into last Wednesday's rampage in San Bernardino came after President Barack Obama vowed to destroy the Islamic State in Iraq and Syria (ISIS) and hunt down its followers, in an address to a jittery nation.

ISIS has praised the California attackers - Syed Farook, 28, and Tashfeen Malik, 29 - as "soldiers" of its self-proclaimed caliphate but did not say they were members of the group.

"As the investigation has progressed, we have learnt and believe that both subjects were radicalised and have been for quite some time," Mr David Bowdich, the FBI's assistant director in Los Angeles, told reporters on Monday. "The question for us is how and by whom and where were they radicalised? "Maybe there's not a by whom. Oftentimes it's on the Internet."

He said the couple, killed in a shootout with the police following the gun attack, went for target practice in ranges around the Los Angeles area, including in the days before the tragedy.

On Sunday, Mr Obama said the attack was an "act of terrorism", and the FBI is investigating it as such. The authorities have, however, said there is no indication the couple were part of an organised larger group or cell.

In addition to two rifles and two handguns used in the attack, investigators found an impressive arsenal at their home.

AGENCE FRANCE-PRESSE