Police said that 32-year-old Shashhank Sharma, a resident of Rohini who rents out vehicles, was arrested this morning.
Sharma claimed to be a member of the People For Animals (PFA), an NGO founded and chaired by Union minister Maneka Gandhi.
The other persons, who were allegedly involved in the attack, have been asked to join the probe.
The three assaulted men -- Rizwan, Ashu and Kamil -- were arrested yesterday and later released on bail.
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The trio were booked for mischief by killing or maiming cattle and under relevant provisions of the Prevention of Cruelty to Animals Act as they were carrying more number of animals in a truck than rules permit.
The men had legal transit papers for transporting the animals.
The second case of voluntarily causing hurt and wrongful restraint was registered against unidentified members of the PFA, according to police officers.
The PFA had denied any association with the incident.