A US National Guardsman is facing charges of attempting to give information to the al-Qa'ida network, shortly before he was due to serve in Iraq.
US defence officials said Ryan Anderson, 26, signed on to extremist Internet chatrooms and tried to get in touch with al-Qa'ida operatives, offering the organisation information on US military capabilities and weaponry.
It is unclear how the government learned of his alleged offer, but authorities began monitoring his communications, the officials said. It does not appear he actually transmitted any information.
Specialist Anderson became a Muslim during the last five years, officials said.
Army Lt Col Stephen Barger said Anderson was being held at Fort Lewis "pending criminal charges of aiding the enemy by wrongfully attempting to communicate and give intelligence to the al Qaida terrorist network".
Barger said Anderson was taken into custody in a joint investigation by the Army, Justice Department and FBI. He was being held at the Fort Lewis Regional Corrections Facility near Tacoma, Washington.
Barger declined to give any details on the arrest, and it was not immediately clear if Anderson had a lawyer.
Anderson is a tank crew member from the National Guard's 81st Armour Brigade, a 4,200-member unit set to depart for Iraq. It is the biggest deployment for the Washington Army National Guard since the Second World War.
Washington State University spokeswoman Charleen Taylor said Anderson was a 2002 graduate with a degree in history. He studied military history with an emphasis on the Middle East.
The brigade has been training at Fort Lewis since November. Eighty per cent of the soldiers - 3,200 - are from Washington state, and 1,000 are from guard units in California and Minnesota.
It includes two tank battalions, a mechanised infantry battalion, engineers, support troops, artillery and an intelligence company.
Anderson is the second Muslim soldier with Fort Lewis connections to be accused of wrongdoing related to the war on terror.
Capt James Yee, 35, a former Fort Lewis chaplain, is accused of mishandling classified information from the US prison for suspected terrorists at Guantanamo Bay, Cuba. Yee ministered to Muslim prisoners there.
There were initial reports that Yee was being investigated as part of an espionage probe, but he was never charged with spying.Reuse content