ST. LOUIS COUNTY, Mo. ( – Investigators are calling for any witnesses of Thursday's officer-involved shooting to come forward, and are looking for one in particular.

Multiple police departments responded to a the scene of an officer-involved shooting near Wellston MetroLink station Thursday afternoon.

According to police, a 29-year-old MetroLink security guard was shot by a suspect, and two St. Louis County officers returned fire. MetroLink officials say the security guard was checking fares when he was shot once in the shoulder. He was armed, but did not draw his weapon, according to officials.

The incident occurred in the 6400 block of Plymouth.

The security officer told the suspect he couldn't get on the train without a fare, but was ignored. The security officer then followed the man, 22, who spun and shot the security officer in the shoulder.

Two St. Louis County officers, who were on an arriving MetroLink train, heard the shots and then chased after the suspect. Police say he tried to get into a stranger's car on Plymouth Avenue,. but was unsuccessful. The car sped away. Police have now released a photo of that vehicle, and are asking that the driver contact them about the incident.

The suspect kept running and officers gave commands which were ignored, police say. The officers then shot him three times in the buttocks.

The guard was transported to an area hospital for treatment, and has been released. The suspect was taken to a hospital in critical condition.

Wellston shooting: Security officer shot in shoulder by suspect. Suspect shot in buttock. Both transported to hospital for treatment.— St. Louis County PD (@stlcountypd) November 17, 2016

A witness who had been on the train waiting to leave the platform recorded cell phone video of several people attending to the guard. He also described seeing the suspect run onto the train to get away after the security guard was shot and then realized St. Louis county police officers were on the train. The two officers were assigned to the Wellston MetroLink station.

"Officers chased after him and he ran around in the bushes," said Tracy Dennis. "When he got in the bushes he did turn around like he was going to do something, charge them or whatever. And they shot him just opened fire and they shot him."

Officers recovered a gun from the suspect.

MetroLink have resumed all services, including at the Wellston MetroLink Station. The trains are operating on schedule without any delays. Previous bus shuttles between the Rock Road, Wellston, and Delmar Loop MetroLink Stations have been canceled.

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