UPDATE 3-Qatari diplomat to leave after US flight scare
* Diplomat was going to visit convicted al Qaeda agent
* Was smoking on plane, made remark seen as threat
* Qatari ambassador calls incident a "mistake"
* U.S. expects diplomat to depart the country quickly (Recasts with purpose of diplomat's trip, adds details)
By Jeremy Pelofsky and Keith Coffman
WASHINGTON/DENVER, April 8 (Reuters) - A Qatari diplomat who caused a security alert on a United Airlines flight was on his way to visit a countryman imprisoned in Colorado for conspiring to help al Qaeda, officials said on Thursday.
Mohammed al-Madadi set off the incident on Wednesday, officials said, when he was caught smoking in a toilet on the plane and made a remark perceived as a threat, sparking initial concerns of another attack on the U.S. aviation system.
Smoking is not allowed on U.S. commercial flights and is subject to stiff penalties but officials said al-Madadi, who holds diplomatic immunity, will not be prosecuted for the smoking or the remark and will leave the United States soon.
Qatar's ambassador to the United States called the incident aboard United flight 663 from Washington to Denver a "mistake" and said the diplomat was traveling on official business.
Al-Madadi was going to the Supermax prison in Florence, Colorado, to visit a Qatari citizen, Ali al-Marri, who is serving a 100-month sentence after pleading guilty a year ago to conspiracy to provide material support to al Qaeda militants, a U.S. official and an embassy representative said.
Al-Madadi, a third secretary at the embassy, was released from custody and would not be charged, said a spokeswoman at Brown Lloyd James, a firm that represents the Qatari embassy.
A U.S. official confirmed no charges would be filed and another senior official said al-Madadi was expected to leave the United States quickly.
That would likely resolve the matter with Qatar, which has been a key U.S. ally and hosts a large U.S. military base.
U.S. officials have ramped up aviation security substantially since Christmas Day when a Nigerian man with explosives in his underwear allegedly tried to blow up a transatlantic flight as it approached Detroit.
Al-Marri admitted last year in his guilty plea that he trained at al Qaeda camps and was instructed to return to the United States where he had been a student.
He had arrived a day before the Sept. 11 attacks on the United States in 2001, which were carried out by al Qaeda militants. A few months later, al-Marri was arrested on fraud charges and later was indicted on terrorism-related charges.
He was due to meet the Qatari diplomat on Thursday morning in a pre-approved visit at the Supermax prison, the spokeswoman at Brown Lloyd James said, adding that al-Marri has received visits by embassy officials approximately monthly.
Al-Madadi also had planned visits with Qatari college students while in the area, she said.
Several other people convicted of terrorism charges are held at the Supermax prison in Colorado. Foreigners in U.S. prisons are permitted periodic visits from representatives of their countries under international law.
In the incident on Wednesday, F-16 fighter jets were scrambled to escort the United Airlines Boeing 757 plane, with 157 passengers and six crew, to Denver International Airport and the diplomat was taken into custody.
"It looks like the individual in question was perhaps smoking in the lavatory and might have made an unfortunate remark" when confronted by airline personnel, a U.S. official said.
Qatar's ambassador to Washington, Ali bin Fahad al-Hajri, said al-Madadi "was certainly not engaged in any threatening activity."
"The facts will reveal that this was a mistake, and we urge all concerned parties to avoid reckless judgments or speculation," al-Hajri said in a statement.
Passengers aboard the flight said no struggle took place and the suspect appeared to be speaking calmly to a U.S. air marshal in the first class cabin.
"I didn't know anything had happened until we landed," said Tim Burney, who was sitting one row in front of the man.
Burney said he heard the marshal telling the man "just be honest with me" and the passenger saying something about being "embarrassed."
The first reports of Wednesday's alert suggested a more serious incident. ABC News, citing federal law enforcement officials, reported air marshals subdued a man who authorities say tried to "light his shoes on fire" on the flight.
Just months after al Qaeda hijackers crashed planes into the World Trade Center and the Pentagon in 2001, Richard Reid, a Briton and admitted member of al Qaeda, was subdued on a transatlantic flight diverted to Boston after attempting to detonate explosives hidden in his shoes. (Additional reporting by James Vicini, Andrew Quinn, Tabassum Zakaria and John Crawley in Washington and Jason Benham in Doha; Editing by John O'Callaghan)