US-born Syed Farook and his Pakistani wife Tashfeen Malik had also travelled to a number of shooting ranges around Los Angeles days before the rampage.
Authorities also found 19 pipes in the couple’s home that could have been turned into deadly bombs.
The FBI, which is now leading the investigation into the shooting, said that in spite of the latest revelations the pair were not part of a wider international plot.
A high speed chase ensued immediately after last week’s shooting, as the murderous couple tried to escape.
The pair then engaged in a shootout with dozens of local police officers, but hours later were pronounced dead inside their vehicle.
Islamic State (ISIS) previously claimed responsibility for the terror attack in San Bernardino on December 2 where people who helped the disabled were having a Christmas party.
Malik had pledged allegiance on Facebook to the jihadis also known as Daesh prior to the shooting.