San Bernardino shooting: Everything we know about Tashfeen Malik

Tashfeen Malik and her husband Syed Rizwan Farook left their six-month-old daughter with his mother before killing 14 people

In this handout provided by the FBI, Tashfeen Malik poses for a photo at an unsepcified date and location (FBI via Getty Images)

Law enforcement officials are attempting to discover what led a couple to shoot and kill 14 people at a social services centre in San Bernardino.

Tashfeen Malik, 29, and her husband Syed Rizwan Farook, 28, left their six-month-old daughter with Farook's mother before perpetrating what is thought to be the worst mass shooting since 26 were killed in Newtown, Connecticut, in 2012.

Malik was killed alongside her husband four hours later in a shootout with police.

On Friday, the FBI said it is beginning to investigate the shooting as an "act of terrorism" as it was revealed Malik had pledged allegiance to Isis in a now-deleted Facebook post.

Tashfeen Malik, left, and Syed Farook died in a shoot-out with police (AP)

The FBI has acknowledged knowing little about Malik. It is unclear when she became radicalised, or who radicalised her.

FBI officers have indicated she and Farook probably "self-radicalised" online.

Authorities discovered a huge arsenal of guns, ammunition and pipe-bombs in the couple's home, suggesting they had been planning additional attacks.

Weapons used by the suspects (San Bernardino County Sheriff's Department/ Facebook)

The remains of a SUV involved in the Wednesdays attack is shown in San Bernardino, California December 3, 2015 (Reuters)

It has been reported Malik moved to Saudi Arabia with her father at a young age, and lived there until 2007.

She is then believed to have returned to Pakistan to attend a pharmacology school at Bahauddin Zakariya university.

An intelligence official said she was a "good student with no religious extremist tendencies". She did not work as a pharmacist in the US.

Malik and Farook met in 2013 on the annual pilgrimage to Mecca, The Guardian reports.

They married shortly after, and she entered the US on a K-1 visa, which is provided for spouses of American citizens.

Security officials say she visited Pakistan again in 2013 and 2014, but it is unclear who she met or where she visited.

Local mosque leaders said she was not well known in the local Muslim community.

Neither Malik or Farook were known to local police or the FBI.

Additional reporting by Reuters