Tashfeen Malik is pictured in this undated handout photo provided by the FBI, December 4, 2015. [Photo/Agencies]

According to the Los Angeles Times report, one of the officials characterized Malik's messages as "her private communications ... to a small group of her friends," accordingto the Times. The official added, "It went only to this small group in Pakistan." The official said they were written in Urdu, an official language of Pakistan.

The second official said Malik "expressed her desire" in one of the posts to become an Islamic militant in her own right, the Times said. Both officials spoke on condition of anonymity, the newspaper said.

US officials have said their investigation has yet to turn up evidence that foreign militants directed Farook or Malik when they stormed a holiday gathering of Farook's co-workers and opened fire with assault-style rifles.

The couple fatally shot 14 people and wounded more than 20 in a rampage the Federal Bureau of Investigation said it was treating as an act of terrorism inspired by Islamist militants.

Farook, the US-born son of Pakistani immigrants, and Malik, a Pakistani native he married last year in Saudi Arabia,were killed in a shootout with police hours after the assault in San Bernardino, 60 miles (100 km) east of Los Angeles.

The FBI has said the couple declared they were acting on behalf of Islamic State. But FBI Director James Comey has said there was no evidence the militant group was aware of them before the attack.