The mother of a terrorist who killed 14 people in a 2015 attack in San Bernardino, California, has agreed to plead guilty to destroying evidence by shredding a map her son may have used to plan the massacre.

Rafia Sultana Shareef, 66, of Corona agreed to plead guilty to a charge of intending to impede a federal criminal investigation by altering, destroying and mutilating records, the US attorney's office announced Tuesday.

The crime carries a potential 20-year federal prison sentence. But under the plea agreement, Shareef is expected to receive no more than 18 months when she is be sentenced in US District Court in Riverside.

Shareef, also known as Rafia Farook, is the mother of Syed Rizwan Farook.

He and his wife, Tashfeen Malik, opened fire with semiautomatic rifles on a Christmas party gathering of Farook's colleagues at the San Bernardino Department of Public Health on December 2, 2015,.

They shot dead 14 people and wounded 22 others before fleeing the scene. They were killed hours later in a shootout with police.

Rafia Sultana Shareef, 66, the mother of San Bernardino terrorist Syed Farook, has agreed to plead guilty to destroying evidence by shredding a map her son may used to plan his attack

Syed Rizwan Farook (left) and his wife Tashfeen Malik (right) shot and killed 14 people during a Christmas party in San Bernardino on December 2, 2015

Farook and Malik fled the scene of the massacre and were killed hours later in a shootout with police. The SUV they were driving is pictured after the confrontation

At the time, Shareef was sharing a Redlands home with her son, his wife and their 6-month-old daughter.

On the morning of the shooting, Farook and Malik left the home, telling Shareef that they were going to a medical appointment, prosecutors said.

Instead, they drove to the Inland Regional Center in San Bernardino and attacked the party.

After Shareef learned that law enforcement had identified her son as a suspect in the attack, she went into his bedroom, 'grabbed at least one document that appeared to be a map, and fed it into a shredder,' the US attorney's office said.

'Shareef admitted that she knew her son had produced the document, and she believed it was directly related to his planning of the IRC attack,' the statement said.

San Bernardino County employees hold up photos of the shooting victims during a candlelight vigil on December 7, 2017

DailyMailTV obtained exclusive photos of Shareef back in December, just days after the four-year anniversary of the massacre

DailyMailTV obtained exclusive photos of Shareef back in December, just days after the four-year anniversary of the massacre.

She was seen arriving at a mosque in Corona with her other son Syed Raheel, 34, and her husband Syed Sr., 70, were there for afternoon prayers.

Approached by DailyMailTV, Shareef refused to discuss her mass murderer son but said she was doing 'okay' four years on.

The mother has always insisted that she did not notice Farook and Malik's increasing radicalization or have any idea of their plans - even though the couple built some of the small bombs later used in the massacre at the home.

In addition to the destruction of evidence charge, Shareef was battling the Justice Department over Farook's $280,000 life insurance policy, of which she was the primary beneficiary.

Insurers Minnesota Life Insurance paid out shortly after the attack but the funds were held by court after the Justice Department attempted to have the cash seized as proceeds of crime.

She said that because she did not know her son was going to commit an act of terror, there is no reason for the money not to be paid.

Meanwhile, Syed Raheel and his wife Tatiana are set to be sentenced later this year after pleading guilty to one count each of conspiracy to commit immigration fraud in January 2017.

They face a maximum sentence of five years in jail and a $250,000 fine.

The immigration convictions relate to a sham marriage arranged by Syed Raheel for his wife's sister Mariyah with terror accomplice Enrique Marquez Jr, 27.

Marquez Jr is currently serving 25 years in federal prison after admitting to purchasing guns that were later used in the attack.