The Muslim husband-and-wife shooters in the San Bernardino, California massacre had been preparing for their terrorist attack at least a year ago and had discussed about it as far as three years ago, law enforcement sources revealed.

The sources told NBC News that Syed Rizwan Farook and Tashfeen Malik had been practicing their shooting skills at a Riverside, California gun range for a year or more before last Wednesday's attack on a holiday office party. The two heavily armed shooters killed 14 people and wounded 21 more at the Inland Regional Center in San Bernardino.

The two also made financial plans for their family after their deaths. Counterterrorism officials told NBC News that Farook and Malik were making preparations for some time to "take care of both grandma and the baby."

The couple lived in a Redlands, California residence with their 6-month-old daughter and Farook's 62-year-old mother, Rafia Farook. They left their daughter with Rafia on the morning of the attack.

Investigators found a $28,500 deposit made to Farook's bank account weeks before the couple staged the horrific attack on Dec. 2.

Another law enforcement official told NBC News that FBI investigators have learned that Farook discussed staging an attack in California as early as 2012.

Meanwhile, the FBI now suspects that Farook's father and mother were also involved in the terrorist plot, WND reported.

The FBI is monitoring Farook's 66-year-old father, also named Syed, because both he and his terrorist son made trips to Pakistan. The father came to the U.S. in 1972 and became a citizen in 1999.

In media interviews following the San Bernardino shooting rampage, the elder Farook tried to distance himself from his son, saying "His views were conservative. My views were liberal."

Farook Sr. said his son had extremist views, hated Israel and had declared that "Islam will rule the world."

Farook's 62-year-old mother, Rafia, is also being investigated for being a possible co-conspirator since she lived in the same Redlands apartment with her son where he stored numerous pipe bombs, firearms and thousands of ammunition rounds before the terrorist attacks.

"The mom definitely has exposure as a potential co-conspirator," the law enforcement source said.

Farook and Malik left their 6-month-old daughter with Rafia before they began their shooting spree. The child is now in the custody of San Bernardino County Child Protective Services.

Authorities held Rafia for questioning after the massacre but released her the next day without charge.