Three police officers were killed and two others critically injured in a shooting in the US state of Pennsylvania, while following-up on a “domestic-related” investigation, authorities have said.

Pennsylvania State Police Commissioner Colonel Christopher Paris said the gunman was fatally shot by the police.

The incident happened just after 14:00 local time (18:00 GMT). Images from the scene showed a police officer being airlifted in a medical helicopter.

The shooting forced a nearby school district in Spring Grove, Pennsylvania – a small town of about 2,500 people – to briefly shelter in place. The district later said the school had not been affected.

Officials said there was no active threat to the public.

The shooting happened in a rural area of York County in North Codorus Township, about 100 miles (160km) east of Philadelphia.

Authorities said officers were at the scene for an investigation that had begun the day before.

Police have not shared many of the details because the investigation was ongoing, Commissioner Paris said.

Pennsylvania Governor Josh Shapiro reached the area on Wednesday afternoon and met the families of the officers killed, local media reported.

“We grieve for the loss of life of three precious souls who served this county, served this commonwealth, served this country,” Shapiro said.

“This kind of violence is not OK, we need to do better as a society.”

Shapiro later ordered all flags in the state to be lowered to half-mast in honour of the three officers killed.

All Pennsylvania state flags will be lowered at all public buildings and grounds throughout the commonwealth.

FBI Director Kash Patel said agents were on the ground providing assistance to local and state partners.

“Our prayers are with the officers, their families, and the entire York County community,” Patel wrote in a statement.