NBC News has learned that San Bernardino shooters Syed Rizwan Farook and Tashfeen Malik spent at least a year preparing for their terror attack, practicing at a local gun range and making financial plans for their family after their deaths.
Two sources said Farook and Malik had practiced their shooting skills at a Riverside, California-area gun range for a year or more before last Wednesday's attack on a holiday office party. They killed 14 people and wounded 21 more with firearms at the Inland Regional Center in San Bernardino.
Counterterrorism officials also 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 Farook on the morning of the attack.
Investigators are examining a $28,500 deposit made to Farook's bank account in the weeks before the Dec. 2 rampage.
A U.S. counterterrorism official said that he didn't have details on the transfer, but that it "would be consistent with them making preparations for grandma and the kid."
"They had purposely thought through that problem," said the official. "There were other indications of preparations."
Reuters reported that the loan to Farook was made by the online lender Prosper. In a statement, Prosper said that the company is "prohibited by law from disclosing any non-public, personally identifiable information regarding any loan originated through our platform."
"All loans originated through the Prosper platform are subject to all identity verification and screening procedures required by law, including U.S. anti-terrorism and anti-money laundering laws," the spokesperson said. "Like all Americans, Prosper is shocked and saddened by recent events in San Bernardino."
The latest revelations come after NBC News reported Monday that federal investigators are interviewing a neighbor of Farook and Malik who bought two of the four guns that were used in the massacre.
That man, Enrique Marquez — who NBC News has learned is an in-law of Farook — had converted to Islam about three or four years ago, according to the son of the imam at the Islamic Center of Riverside.
Marquez, along with Farook and Malik, attended the mosque, officials have said. In 2014, records show, Marquez married a woman named Mariya Chernykh — the sister of Tatiana Farook, who is married to Syed Rizwan Farook's brother, Syed Raheel Farook.
Both Tatiana and Syed Raheel Farook were witnesses at the ceremony, according to public records.
Enrique Marquez and Mariya Chernykh also listed their home address as the same one belonging to Syed Raheel Farook and his father.
Muhammed Kuko, the son of the imam at the Islamic Center of Riverside, told NBC News that Marquez went to Friday prayers there in addition to working at its bookstore.
He said he heard Marquez was a gun hobbyist, and he came across as mild-mannered, smiling and helpful — and Kuko never heard him express any extreme or conservative views. He said he would be shocked if Marquez was knowingly an accomplice to the massacre.
Kuko added that Farook was also reserved and mild-mannered, and sometimes liked to play on the mosque's basketball court.
Marquez has not been named as suspect in the case, and investigators say he has been cooperative. Law enforcement sources said he bought the two semiautomatic rifles used in last week's attack in 2011 and 2012.
All four guns were purchased legally, officials have said.
NBC News' Pete Williams, Richard Esposito and Hannah Rappleye contributed to this article, which first appeared on NBCNews.com.