The M1A1 Abrams tank was built to withstand a heavy onslaught. But at the end of August, an Abrams tank on patrol in Baghdad was disabled by a mysterious projectile that has left the Army eagerly searching for an answer as to why one of its most invincible pieces of machinery was stopped by something that created only a pencil-size hole.

The incident, first reported by the Army Times newspaper in late October, is of interest on Arsenal Island because an Arsenal-based technician working in Iraq examined the tank and wrote an unclassified report detailing the incident, which confessed, frankly, that officials were mystified as to how it happened.

“The unit is very anxious to have this ‘SOMETHING’ identified. It seems clear that a penetrator of a yellow molten metal is what caused the damage, but what weapon fires such a round, and precisely what sort of round is it? The bad guys are using something unknown and the guys facing it want very much to know what it is and how they can defend themselves,” Terry Hughes said in his report, according to the Army Times.

Don Jarosz, an official in Warren, Mich., where Hughes’ command is headquartered, said Friday that the report, while unclassified, was not available. In addition, a tight lid is being kept on the command’s investigation into the incident for security reasons.

Hughes is a logistics assistance representative, a troubleshooter who works in the field connecting combat units with the commands that supply their weapons. He works for the Tank-automotive and Armaments Command, which is based in Michigan but whose logistics assistance representatives are controlled out of the tank command’s office on Arsenal Island.

The incident has not received a lot of attention in the news media, but reports have suggested the Army is keenly interested in solving the mystery. There were no serious injuries as a result of the incident.

A military analyst with the Globalsecurity.org think tank says that, as of a week ago, there still was not an answer.

Speculation has been that a specially designed rocket-propelled grenade may have been the culprit or that terrorists got their hands on a special armor-piercing round, analyst Patrick Garrett said. “There’s also been speculation this was some insanely lucky shot,” he said Friday.

It is not unheard of for one of the 67-ton Abrams to be disabled, he said. An M1 Abrams tank was destroyed and two soldiers were killed by a mine in Iraq at the end of October.

Still, it is unusual for the tanks to be felled, and the fact that it would happen with such an apparent precision-type strike is troubling. The Abrams was designed for Cold War combat.

“They were designed to deal with the Soviet juggernaut,” Garrett said. They are not supposed to be stopped by a pencil-thin projectile. “That’s what’s very confusing about it,” he added.

How long the military’s investigation will take and whether it will be something the public ever knows about is also unclear. Garrett said the Abrams tank is one of the military’s most sensitive pieces of equipment, and the Army probably will not be eager to share information about how one of them was crippled.