A metal drum found in northern Iraq that initially tested positive for nerve and blister agents might instead contain rocket fuel, according to new tests, a U.S. chemical weapons expert said Monday.

More tests were planned in the coming days on the 55-gallon drum, said Lt. Col. Valentin Novikov, the chief chemical weapons officer of the 4th Infantry Division (search), the unit which found the site.

Novikov's comments raised the prospect that the discovery was the latest in a series of false alarms as U.S. troops try to find the remains of Saddam Hussein (search)'s suspected programs for biological, chemical and nuclear weapons.

The suspicious barrel was among 14 barrels found in an open field near the Tigris River town of Baiji, among mounds of earth that hid missiles and missile parts. U.S. troops performed an initial test and found indications the barrel may contain the nerve agent cyclosarin and a blister agent that could be a precursor of mustard gas.

By design, initial test procedures favor positive readings, erring on the side of caution to protect soldiers.

Two teams of experts were brought in this weekend for additional testing.

One team conducted three tests, but the tests "were not totally conclusive," Novikov said.

The second team, a specialist Mobile Exploitation Team, "suspects that it might be rocket fuel," Novikov said.

That team is expected to return to the site in the coming days for further tests.

"There is a chance that it could be chemical weapons, but we don't know for sure," Novikov said, speaking outside of the 4th Infantry headquarters, a former palace in Saddam's hometown of Tikrit, near Baiji.

Also found at the site were two unmarked vans that soldiers first suspected to be mobile chemical laboratories. Inside the vans were three cylinders for mixing liquids and a dosage chart in English and Russian. Green camouflage netting was draped across the front of one of the vans.

Novikov, however, said the vans "could be" a rocket fuel mixing station.

Near the site was a low, brown sandstone building that had 150 gas masks that are of a higher quality that those usually used by Iraqi soldiers.

The initial tests on the barrel were conducted late Friday by Lt. Valerie Phipps and Pfc. Jeremy McCullough, chemical warfare experts with the 1st Squadron of the 10th Cavalry Regiment.

All three of their tests pointed to nerve or blister agents. Afterward, Phipps and McCullough left the area and burned their chemical warfare suits for fear that they were contaminated.

There have been numerous false reports that coalition forces have turned up chemical or biological weapons.

Mustard agent burns skin, eyes and lungs, while exposure to high amounts cyclosarin may lead to loss of muscle control, twitching, paralysis, unconsciousness, convulsions, coma, and death within minutes.

Abrams tanks and Bradley fighting vehicles ringed the 1.5-square-mile field Sunday and Monday, watching for intruders. Troops had permission to shoot to kill if anyone entered the area, which was near the Tigris River about a mile outside Baiji.