The family of a Tucson man being held on terrorism charges says in a new statement that he is autistic, incapable of caring for himself and couldn't carry out any of the attacks he is suspected of plotting.

The parents of Mahin Khan, 18, issued the letter to Tucson TV station KGUN9 on Thursday. An attorney for Khan said the statement was authentic but that he did not have a copy.

Khan, who turned 18 a month before he was arrested in his family's Tucson home, is accused of plotting a terrorist attack on a motor vehicle office in metro Phoenix. He will remain jailed without bond until his trial.

Khan's parents say in the statement that their son has suffered from mental health issues and autism since early childhood and that he has been through a long inpatient psychiatric evaluation under the supervision of the FBI.

"We have been fully cooperating with the FBI over the last three years to ensure that our son gets the help he needs," the parents wrote. "Although he is now 18 years old, Mahin's mental age according to mental health professionals is less than 13."

The Khans also said their son isn't able to function on his own and doesn't pose any real threat, adding that he doesn't have the mental capacity to carry out any of the acts he's accused of planning.

Khan has pleaded not guilty to state charges of terrorism, conspiracy to commit terrorism and conspiracy to commit misconduct involving weapons.

But a Maricopa County Superior Court judge on Wednesday granted a request by prosecutors to keep Khan in jail until his trial over concerns that he poses a public safety threat. Khan is being held in Phoenix.

Authorities say Khan wanted to attack a motor vehicle office in Mesa, Arizona, and instructed an undercover FBI employee to start building homemade grenades.

They say Khan wanted to inspire an insurgency in the United States to carry out the sort of attacks that had occurred in Paris and Brussels and had online exchanges with a person believed to be a member of the Islamic State terrorist group.

Khan expressed a desire to attack a Jewish community center in Tucson, police said. Authorities also said they found a document in a search of his home that showed he wanted to attack a military recruiting center and an LA Fitness location.

But the Khans say they feel their son is being stigmatized because of his mental illness. "We strongly feel that the criminalization of individuals with mental illness is one of the most significant challenges in our punitive criminal justice system and Mahin undoubtedly falls into this category," they wrote.