Either a very elaborate prank, or a test of security at Indianapolis International Airport: Inactive explosives found at airport.

A suspicious item found in a security checkpoint at Indianapolis International Airport this morning caused a concourse of the airport to be closed for about an hour, delaying several flights, according to a report from the Indianapolis Airport Police.

An unidentified passenger reported the suspicious package, which appeared to be an improvised explosive device, after finding the item in a stack of gray plastic trays used to send personal belongings through the X-ray machine, the report stated.

Police shut down the checkpoint and adjoining concourse D from 5:10 to 6:20 a.m. A perimeter was set up and a bomb squad from the Indianapolis Metropolitan Police Department rendered the device safe.

The suspected bomb appears to be a simulated explosive device, the report stated. The device included a battery, wires, a switch, and a plastic bag containing a modeling-clay-like substance labeled as a semtex imitation. Semtex is a plastic explosive often used in terrorist attacks.

According to the report, a second plastic bag also contained a fake explosive consisting of a small amount of a liquid labeled helix and a powder-filled tube. The bag was marked with a label from manufacturer S.E.T.D. Law Enforcement Training Center in Stamford, N.Y., the report stated.

Police called the manufacturer and were told that the company did manufacture the simulated helix but did not make the semtex imitation.

(Hat tip: ndoyle.)

UPDATE at 9/26/07 11:18:34 am:

Here’s the explanation: Training supplies cause Indianapolis airport evacuation.

INDIANAPOLIS - Security officials evacuated a concourse at the Indianapolis International Airport Wednesday after inactive explosives inadvertently left behind by a federal employee were found at a security checkpoint.

The items, including a vest with “inert training explosives,” were found in a tray on a conveyor belt at a security checkpoint leading into Concourse D, the Transportation Security Administration said.

TSA spokeswoman Lara Uselding said airport police determined that the suspect items were “components used by TSA to train its officer workforce” and that passengers were never in danger.

“A TSA training officer had used the components for training purposes overnight and then inadvertently left them at the checkpoint,” Uselding said in a statement.

And the level of casual incompetence this reveals is almost as disturbing as a real dry run.