TUCSON, Ariz (Reuters) - A security incident that placed Davis-Monthan Air Force Base in southern Arizona on lockdown ended after a thorough search of the facility found no gunman or weapon, the base commander said on Friday.

Colonel John Cherrey said the base was locked down from 10:30 a.m. local time through 4 p.m. when a "room-by room, floor by floor" search of a building where a gunman was reportedly sighted earlier the day was completed.

"Local and federal law enforcement personnel, in concert with base security force personnel, conducted a sweep and secured the building where the alleged gunman was reported," Cherrey told a news conference.

"No gunman or weapon was found and the building was declared secure and an orderly release of personnel was initiated," he added.

Earlier on Friday, Sergeant J. Russell Martin said the base was put on lockdown after "unconfirmed reports" of an armed man seen near an old dormitory on the base.

The Davis-Monthan base is about five miles southeast of downtown Tucson, and is home to the U.S. Air Force's 355th Fighter Wing, which Col. Cherrey commands.

The scare at the base came after the Pentagon earlier this month raised the alert level at U.S. military bases to guard against any Al Qaeda plot timed for the tenth anniversary of the September 11 attacks on New York and Washington.

Potential domestic threats to U.S. military facilities were also highlighted by a fatal shooting spree at a Texas military base two years ago, when an Army psychiatrist opened fire on a group of soldiers.

Major Nidal Malik was charged with the rampage at Fort Hood in November 2009 that killed 13 people and wounded 32 others, among them troops preparing to deploy to war zones in Iraq and Afghanistan. He is due to go on trial next March.

Later on Friday afternoon, parents were allowed on to the base to collect their children from two schools there, a spokeswoman said.

(Reporting by David Schwartz in Phoenix, Molly O'Toole in Washington and Alex Dobuzinskis in Los Angeles; Writing by Tim Gaynor; Editing by Cynthia Johnston and Peter Bohan)