The wife of Orlando nightclub gunman Omar Mateen knew her husband was going to attack and had assisted him, prosecutors said, seven months after 49 lives were lost in the deadliest mass shooting in the US history.

"She knew he was going to conduct the attack," assistant US Attorney Roger Handberg told the federal court in Oakland on Tuesday, one day after Noor Salman was arrested at her parents' home in the San Francisco suburb of Rodeo, Xinhua news agency reported.

She was charged with aiding and abetting her husband, who pledged allegiance to the Islamic State terrorist group during the attack on June 12.

Investigators believe Salman acted of her own free will and knowingly took steps to obstruct the probe into the massacre, a CNN report quoted law enforcement officials as saying.

Months before the attack, Mateen added the name of his wife to his life insurance policy and made sure she had access to his bank accounts. He also bought his wife an expensive piece of jewellery, said the report.

Two hours after the attack started, Mateen texted his wife at 4 a.m. and asked her whether she had heard any news about the shooting. At one point, Salman sent a text to Mateen saying that she loved him, a law enforcement official said.

Her family, meanwhile, maintained that she had no idea that her husband was going to attack the popular Pulse nightclub.

"Noor Salman had no foreknowledge nor could she predict what Omar Mateen intended to do that tragic night," family attorney Linda Moreno of Tampa, Florida, said.