NEW DELHI (Reuters) - Two men on motorcycles threw a grenade at a Sikh religious gathering of about 250 people in Amritsar city in the northern Indian state of Punjab on Sunday, killing three and injuring around 20, police said.

Initial reports suggest that two people armed with pistols came to the venue, said provincial police chief Surinder Pal Singh. “There was a religious congregation over here and they lobbed a grenade,” Singh added.

It was not clear if the incident was linked to a separatist campaign by Sikh militants.

Some Sikhs began agitating for a separate homeland in India in the 1970s, before their movement petered out in the 1990s. India believes there has been an attempt to revive it over the past few years.

Punjab Chief Minister Amarinder Singh said such Sunday congregations were common among Sikhs throughout the state and that is why there was not much security there.

The state borders Pakistan, India’s arch enemy.

“We will get to the bottom of this,” he told the Republic TV news channel, adding that in future all such congregations will be provided security cover.

The chief minister said he suspected the involvement of Pakistan’s main spy agency in the attack.

India believes Pakistan supports Sikh separatists, an accusation Islamabad denies.