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, BBC reported.

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, according to BBC