The Muslim husband and wife behind the mass shooting in San Bernardino had gone to great lengths to keep their plans secret from friends and family and also began erasing their digital footprint a day in advance of the deadly attack.

Law enforcement sources involved in the investigation of the deadly attack, which killed 14 people and injured 21, said the couple began deleting email accounts, disposing of hard drives and smashing their cellphones the day before the attack at a holiday party held for local government employees.

Investigators had edged closer Thursday to the conclusion that Malik and her husband, Syed Rizwan Farook, were radicalized by Islamist extremists either in the U.S. or during trips to the Middle East, including Saudi Arabia.

It remained unclear Friday whether the couple may have been self-radicalized or that others may have influenced their actions as officials scrambled to re-create the travels, contacts and lifestyle of Farook, a 28-year-old U.S. citizen of Pakistani descent, and his 27-year-old Pakistani wife.

The FBI, which is handling the massacre as a counter-terrorism investigation, though officials have not classified the attack as such, declined Friday to comment on the reports.

“As this remains an ongoing investigation, we are unable to provide additional information at this time,” a spokeswoman said. “The FBI continues to work in close cooperation with our federal, state, and local partners to investigate this matter.”

On Friday, the landlord who owned the house the couple rented allowed reporters to enter the apartment. MSNBC aired footage of reporters rummaging through the couples’ personal belongings, including photos, credit cards, and a Quran.