Indian police said on Saturday they had arrested one man on suspicion of murdering two Muslim men whose charred bodies were found in a car after they were abducted for alleged cow smuggling.
Cows are considered sacred by Hindus and their slaughter is illegal in many Indian states.
Vigilante squads that roam motorways to stop the animals from being transported across state borders have proliferated since Prime Minister Narendra Modi's Hindu nationalist Bharatiya Janata Party came to power in 2014.
The bodies of the two men were discovered in a burnt vehicle in northern Haryana state on Thursday morning, a day after they went missing.
Both men were from neighbouring Rajasthan state and their families named five men belonging to the Hindu hardline Bajrang Dal group in a criminal complaint to police after their disappearance.
"We have arrested one accused so far and we are searching for others," police officer Shyam Singh said.
Rajasthan police said the arrested man was a taxi driver and was also involved with cow vigilante groups.
Rajasthan Chief Minister Ashok Gehlot condemned the killings on Twitter and said strict action would be taken against the accused.
Critics have accused Mr Modi's government of turning a blind eye to vigilante attacks on minority Muslims in the name of cow protection.
Rajasthan and Haryana are among the states that ban cow slaughter, and authorities also require anyone transporting the animals across state borders to have a permit.