Indian authorities investigating the assassination last week of prime minister Indira Gandhi have contacted Interpol to help them locate a third man believed to have played a key role in the murder and who is now thought to be in Canada.

R.C. Kohli, deputy police commissioner in charge of the investigation, today said the authorities have been in touch with the Paris-based International Police Organization, but he would not elaborate on the investigation. However, sources close to the management of the Sikh temple here said the two Sikh security guards who gunned down the 66-year-old leader Wednesday took an oath from a Sikh spiritual leader, known only as Giani, to kill her.

This account could not be verified independently, however. Kohli, asked about the alleged conspiracy, had no comment.

Giani is believed to be a follower of Jarnail Singh Bhindranwale, the militant preacher who was killed when the Army stormed the Golden Temple, Sikhism's holiest shrine, in the northwestern state of Punjab this summer. The storming of the temple has been condemned bitterly by Sikh leaders.

According to sources at the New Delhi Sikh temple, Giani was headed for Canada, where he is believed to have relatives. Press reports quoting police sources said he left India 12 days ago.

There are large communities of Sikhs in Canada, particularly in Winnipeg, Toronto and Vancouver, and the number of Sikhs living in North America has been put at 10,000, by some estimates.

After the invasion of the Golden Temple by the Indian Army, Canadian press reports quoted Sikh separatists as saying they had begun to collect funds and a list of volunteers for the assassination of Indira Gandhi.

Officials in Canada's Department of External Affairs could not confirm reports that Giani was being sought in Canada.

Following last week's assassination, much attention has focused on the sectarian violence that erupted and the ability of Rajiv Gandhi to lead the country, but little has been said about the apparent security lapses that allowed the elder Gandhi to be gunned down, or the subsequent investigation.

One of the alleged assassins, Beant Singh, a subinspector with the Delhi police, was shot dead by other security guards at the scene. His father and three brothers have been taken into custody for questioning, according to Indian news agency reports.

The other alleged assassin, Satwant Singh, a constable with the Delhi police, is reported to be recovering from his gunshot wounds but is still "not out of danger and nothing can be said until Sunday," doctors of the Lohia Hospital here said today. Another press account earlier this week said commandos armed with light machine guns were guarding his hospital room.

Today, a report in the normally reliable, conservative Statesman said intelligence agencies have asked the doctors treating Singh to "casually ask him about the murder conspiracy."

Citing "sources close to the investigating team," the report said the doctors have agreed to cooperate with the authorities in obtaining information from Singh, who has not recovered enough to be interrogated by police. His relatives have also been taken into custody, Indian news agencies have reported.

Official sources have said Satwant Singh had been on leave during the past two months in the Gurdaspur district of Punjab, bordering Pakistan, a stronghold of extremist disciples of Bhindranwale, and returned to the capital a few days before the shooting.

Local press accounts also have given varying accounts, almost all of them without sources, of a broader conspiracy involving police officials and in one case, a Sikh major general in the Army. The Indian Defense Ministry has denied that account.

Meanwhile, Rajiv Gandhi's newly formed Cabinet has appointed a commission of senior government officials, none of whom have been publicly identified yet, to investigate the assassination.

Immediately after the shooting, Indian news agencies reported that the entire security staff at the prime minister's residence had been disbanded and replaced with intelligence agents of the central government.

The prime minister's security staff was formed about a year ago and was made up of about 150 men -- selected regardless of their religious adherence -- from various contingents of India's paramilitary forces.

Her security was tightened about a year ago when she received a letter from Sikh terrorists threatening her and her immediate family.