Four people were injured, two critically, in an axe attack on a suburban train outside Paris early Monday, according to police.

The violent altercation erupted around 8:00 a.m. (0700 GMT) on an RER express train stationed at Ozoir-la-Ferrière, southeast of Paris. One victim lost a hand, while another suffered a skull fracture, the police source said, speaking anonymously. Two others sustained less serious injuries.

Authorities reported that multiple individuals, including minors, were involved in the fight, although no additional details on the attackers or their motives were provided.

Since the train was stationary at Ozoir-la-Ferrière station, the attack did not disrupt service on the RER E line, which links Paris with its suburbs, stated the state rail operator SNCF.

The incident has prompted increased security measures across the suburban rail network. Police are continuing to investigate the circumstances surrounding the altercation.