The military command responsible for the defense of North American airspace scrambled fighter jets in response to unverified reports of an airborne condensation trail, or contrail, moving from the Caribbean to the United States, defense officials said yesterday.

Lt. Col. Michael Humm, a Pentagon spokesman, said the incident happened Wednesday and that the North American Aerospace Defense Command in Colorado Springs was continuing to investigate.

The reported contrail stirred concern because of the possibility that it could have indicated the presence of an unauthorized jet aircraft in or approaching U.S. airspace.

In the aftermath of the Sept. 11 attacks, the Pentagon has taken greater precautions to monitor U.S. airspace.

A contrail is created by vapor from a jet engine in the presence of cold air.

The jets that were scrambled to attempt to intercept and identify the source of the contrail found nothing, said Lt. Cmdr. Curtis Jenkins, a NORAD spokesman.

He said NORAD had developed no new information since the initial report at 4 p.m. EST on Wednesday.

NORAD is reviewing data from its tracking radars in search of evidence, he said.

A Pentagon statement said NORAD received unverified reports of "what appeared to be a contrail of unknown origin," originally in the vicinity of the Turks and Caicos Islands near the Bahamas.

"Initially, it was reported to be heading northwestward toward the United States," the statement said. "Commercial airline pilots later reported the contrail over Florida and later over Indiana. Thereafter, no other sightings were reported."

The reported contrail was never verified by visual or radar contact, the Pentagon statement said.