A suspect is in custody after four people were fatally shot while riding a Labor Day morning train in a Chicago suburb, authorities said Monday.

The lone gunman was in custody Monday evening, according to police.

The Forest Park Police Department said it received a 911 call shortly before 5:30 a.m. reporting that three people appeared to be shot on a train at the Forest Park Chicago Transit Authority Blue Line Station, about 10 miles west of downtown Chicago. Officers cleared the station and found four shooting victims. Three died at the scene, while the fourth person was taken to a local hospital and later pronounced dead, according to police.

Forest Park Deputy Police Chief Christopher Chin told USA TODAY all four victims appeared to have been sleeping on the early morning train when they were shot. The first three people were spread out across one train car, he said, and the last person was in another train car.

"It's believed to be random," Chin said.

The shooter initially fled, and a suspect was later identified through video surveillance, authorities said. Chicago police located the suspect on a CTA Pink Line train, who was taken into custody. A firearm was also recovered, Forest Park police said.

Police did not publicly identify the victims or the suspect. A motive was not immediately clear, but Chin said it was an “isolated incident.” He added the lone suspect was taken into custody around 7 a.m. Monday and is expected to be charged by Tuesday evening, but authorities have 48 hours to formally file charges.

“It’s a horrible tragedy that four people are dead on Labor Day weekend,” Forest Park Mayor Rory Hoskins said at a news briefing. He noted that police respond to the train station more than any other area in town, but rarely for a mass shooting.

According to a database by USA TODAY, the Associated Press and Northeastern University, more than 3,000 victims have been killed in 602 mass killings since 2006.