Police have identified the gunman who killed a patient at a Covid-19 field hospital in Pathum Thani early Thursday morning as a former soldier.

Kawin Saengnilkul, a 23-year-old from Thanyaburi district, was thought to be fleeing to the South in a white Isuzu pickup.

He served as a private in a special warfare unit in Lop Buri province, but was dismissed from the service in November 2019.

He had applied to join as a private because of his enthusiasm for weapons, police said.

Pathum Thani police also alleged that before the attack at the field hospital, the gunman had shot dead an employee at a convenience store in the Phahon Yothin area of Bangkok about 2am.

The incident happened at the Princess Mother National Institute for Drug Abuse Treatment on Phahon Yothin Road in Thanyaburi district.

Media reported that the gunman, wearing a camouflage outfit and red beret, fired a gun shot to break open the locked front glass door of the institute, which functions as a field hospital for Covid-19 patients, about 3.30am and entered the building.

That was when the patient aged about 50 exited a toilet and was shot dead.

The gunman then moved to a ward and fired many more shots before escaping on a pickup truck. The subsequent shots did not hit anyone.

Police were hunting the gunman.

An army spokesperson earlier said the gunman was not a soldier.

CCTV screenshot of the gunman. (Photo supplied)