HOUSTON -- A passenger was shot to death after an argument on a METRO bus in southwest Houston on Wednesday, police told KPRC Local 2.

Houston police said the shooting happened on the No. 82 inbound bus on Westheimer Road near Royal Oaks Club Drive at about 11:45 a.m.

Detectives said two passengers exchanged words and then one pulled a gun.

"The suspect was on the bus making his way through the center aisle when he brushed up against the victim," said Capt. Dwayne Ready with the Houston Police Department.

Witnesses said they were scared for their lives.

"When the guy was shot, he dropped. The guy that did the shooting just stood there with a gun in his hand, with the gun smoking, like he's getting ready to start shooting again. So, we didn't know what was going to happen," witness Gwen Guidry said. "(The gunman) said the guy threatened to kill him."

Guidry, a nurse, tried to save the victim, but the single gunshot wound may have punctured his lung, officials said.

"I did what I could and it wasn't happening," she said.

The gunman was arrested without incident and did not try to harm anyone else, according to authorities.

Police said 30 passengers were on board. One of the other passengers had chest pains after the shooting and was transported to a hospital, officials said. The passenger is expected to be OK.

The victim's identity was not released.

The eastbound lanes of Westheimer were closed to traffic while police investigated the scene.

METRO officials said passengers should not be scared to get on its buses because METRO transports 330,000 passengers every day and this was an isolated incident.