By David Schwartz

PHOENIX (Reuters) - An Arizona teenager jailed on charges of plotting to bomb a public building suffers from autism, with the mental capacity of someone younger than 13 and a "limited ability to function on his own," his parents said on Thursday.

Mahin Khan, 18, who, according to the Federal Bureau of Investigation, professed to be an "American jihadist," was arrested on July 1 and has been charged in a three-count indictment with terrorism, conspiracy to commit terrorism and conspiracy to commit misconduct involving weapons.

Khan, a resident of Tucson, is accused of planning to stage an attack with bombs and other weapons on a Phoenix-area state motor vehicle office.

He pleaded not guilty last week, and was ordered to remain held without bail on Wednesday at the end of a two-day hearing in which a Maricopa County Superior Court judge found that he posed a "substantial danger to the community" if released.

But in a statement posted online by Tucson television KGUN-TV and verified as authentic by defense lawyer Robert Ditsworth, Khan's parents said their son lacks "the mental capacity to carry out the horrendous acts he is accused of planning."

"Since early childhood, Mahin has suffered with mental health issues including cognitive disorder and developmental delays, namely autism," they wrote in the statement posted online.

Although Mahin is now 18, mental health professionals have put his mental age at "less than 13," he parents said, adding: "We believe that he does not pose any real threat to society."

The statement, signed by "Mr. & Mrs. Khan," also said Mahin previously underwent "extensive inpatient psychiatric evaluation under the directive and supervision of the FBI," and that review "documented the extent and severity of his mental health."

An FBI agent testifying at this week's court hearing, Ben Trentlage, said Mahin first came to the agency's attention when he was 15, and that his family was urged then to seek mental health treatment for the boy. Trentlage did not elaborate.

Authorities have said the case against Khan stems from contacts Khan had with undercover FBI operatives, and that he communicated with someone he believed was an Islamic State militant seeking components for pressure-cooker bombs and pipe bombs.

Trentlage testified that a search of Khan's home uncovered handwritten attack plans, three pressure cookers and what may have been a crude Molotov cocktail, or firebomb.

If convicted, Khan faces a maximum penalty of life in prison.

(Writing by Steve Gorman; Editing by Jonathan Oatis)