A late-night police pursuit ended with a drug bust and the discovery Monday of thousands of pounds of marijuana concealed in a railroad tank car that entered the U.S. from Mexico this month.

Police Detective Gus Villanueva said numerous bundles of pot were removed from the gooey inside of the petroleum rail tanker by late morning and many more packages were still inside.

The retrieval operation in a rail yard southeast of downtown Los Angeles was halted at about 10 a.m. because something suspicious was found inside and a bomb squad was summoned, Villanueva said.

The bomb squad set a charge and detonated the small cylindrical object at 5 p.m.

"The explosion created enough destructive force to destroy the suspicious cylinder but not to destroy the integrity of the container," said Fire Capt. Steve Ruda, referring to the tank car.

He said the squad was in the process of determining what the object was and whether it was safe for crews to continue removing the marijuana bundles.

The marijuana discovery occurred after officers tried to halt a vehicle traveling erratically east of downtown late Sunday and the driver refused to stop. The chase ended near the rail yard in Commerce, where three men jumped out of the car and were quickly arrested.

Officers noticed an oily substance on the suspects' clothing and found bundles of marijuana coated with oil in the car.

The officers were then led to the nearby tank car, Villanueva said.

The tanker was moved about a half-block to a safer area, and additional bundles were found inside.

Investigators used Fire Department ladders to reach the hatch atop the tanker. Marijuana removed from inside was lowered to the ground by bucket.

Thirty-eight bundles of marijuana were removed from the tanker before the operation was halted. Authorities said a similar number remained inside.

Two firefighters who experienced rapid heartbeats while involved in the operation were taken to a hospital for precautionary exams.

The tank car, owned by the Chicago-based Union Tank Car Co., crossed the border into the United States on July 4 and reached Commerce on July 10 with documentation marking it as empty, said Aaron Hunt, spokesman for Union Pacific Railroad's western region. The car's point of origin was not confirmed, and a phone message left with Union Tank Car was not immediately returned.

Hunt did not know if it had been inspected upon entry and arrival.

"The information I have to date is this is very unusual," he said. "This doesn't happen often for us, not at all."

Commerce is a small industrial city with many rail facilities.