An Indian railway security official has been arrested after he allegedly opened fire and killed four people on board an express train between Mumbai and Jaipur.

Here is what to know about the incident:

The accused was identified as Chetan Singh, a constable in India’s Railway Protection Force (RPF), according to police.

The 33-year-old early on Monday allegedly used his service rifle to fatally shoot four people on the Jaipur-Mumbai Central Superfast Express. He fired 12 rounds from the automatic weapon, reports in Indian media said.

The suspect first allegedly shot at his senior colleague, RPF Assistant Sub-Inspector Tikaram Meena. Some reports said the incident took place after a heated argument between the two took a communal turn, but this could not be independently confirmed.

The accused then allegedly opened fire at a passenger who was in the same compartment, according to the Indian Express.

Singh later crossed into two separate coaches where he allegedly killed two other passengers.

He then tried to stop the train and got off near Borivali, a town in Maharashtra state, where he was apprehended by police.

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Meena, a 57-year-old man from Sawai Madhoper village in Rajasthan state, was due to retire soon from the RPF after 33 years of service, reports said.

Indian media also reported that the other three victims were identified by police as Abdul Kadar, 64, Asgar Abbas Ali, 48 and Syed Saifullah. They were all reported to be Muslim.

Ali, a bangle seller, was the last man to be allegedly shot by Singh.