Investigations into the assassination of prime minister Indira Gandhi by two Sikh security guards are getting under way amid reports of a wider conspiracy and speculation about the possible involvement of militant Sikhs living abroad.

Indian intelligence officials have told local newspapers that other unidentified persons knew of or were involved in the plot and that a "substantial amount" of money changed hands before the assassination. However, no proof of the allegations has been produced yet.

Much of the investigations will depend on the statements of the surviving suspect, Satwant Singh, a 21-year-old police constable assigned to security duty at the prime minister's residence. Witnesses said he used a Sten gun to fire more than 20 bullets into Gandhi's body after another guard, police Subinspector Beant Singh, 36, had opened fire on her with a .38-caliber revolver as she was walking toward her office in the heavily guarded residential compound.

Beant Singh subsequently was killed by other security guards, and Satwant Singh was seriously wounded.

Satwant Singh is undergoing interrogation after having been moved early yesterday from a heavily guarded hospital to an undisclosed location and formally placed under arrest. Although swathed in bandages and said to be paralyzed from the waist down by one of several bullet wounds, Satwant Singh was declared "medically fit" for questioning.

Also reportedly being questioned are three other police subinspectors described as close to the assassins, a police constable who bunked next to Satwant Singh in a barracks, relatives of the two men and members of an Indo-Tibetan Border Police company who were among the prime minister's security guards and took the assassins into custody.

The main responsibility for ferreting out the facts in the assassination has been assigned to a special police investigation headed by S. Anand Ram, chief of the Central Industrial Security Force. The Home Ministry has designated him a special police commissioner with authority to choose an independent panel of investigators from the Central Bureau of Investigation and other Indian police and intelligence agencies.

Anand Ram has submitted a list of a dozen senior police and intelligence officials to the Home Ministry for approval and is expected to launch his investigation formally next week, authorities said. Among the senior investigators nominated is a representative of the Research and Analysis Wing of the Cabinet Secretariat, the agency that handles foreign intelligence-gathering. The agency's help is needed to establish whether the assassins had any foreign connections or assistance.

The senior investigators are to be assisted by about 100 subinspectors and other staff, authorities said.

In addition, the central government has appointed a senior Supreme Court justice, M.P. Thakkar, to conduct a separate judicial inquiry into the assassination.

Demands also have been made for investigators to look into the failure of police to prevent a rampage of anti-Sikh rioting, arson and murder that gripped the capital and other parts of India following the assassination on Oct. 31. The government has put the official death toll at 1,277, with half the killings in Delhi. Thousands of other Sikhs were forced to flee their homes by mobs of enraged Hindus.

On the face of it, the assassination was an inside job led by one of Gandhi's most trusted bodyguards and motivated by a desire to avenge the Indian Army's attack on Sikh militants in the revered Golden Temple in June. Yet a number of mysteries remain to be solved.

Among them are the connections, if any, between the two accused assassins and Sikh extremists in India's troubled Punjab State and abroad, the apparent security lapses that allowed the plot to go undetected and the assassination unhindered, and the circumstances of the subsequent shooting that killed Beant Singh and wounded Satwant Singh after they had surrendered.

Spokesmen for the prime minister's office and the Foreign Ministry have declined to comment on Indian press reports that Beant Singh is related to a senior Indian diplomat, Harinder Singh, who quit his post at the Indian Embassy in Oslo and publicly declared his support for Sikh separatists after the Army's "Operation Bluestar" raid on the Golden Temple complex in Amritsar, the Sikh holy city in northwestern Punjab State.

According to these accounts, Beant Singh accompanied Indira Gandhi on visits last year to Scandinavian capitals and London, where he met with Harinder Singh.

In Oslo, United Press International reported that Harinder Singh denied any connection with the assassination.

"I categorically deny any allegation that I had anything whatsoever to do with the assassination of Mrs. Gandhi," he said in a telephone interview from his Oslo home.

"This is a blatant lie," said the former diplomat, who asked for political asylum in Norway when he resigned his post. "Since my asylum request, they have leveled all sorts of charges against me," he said.

The Indian Embassy in Oslo refused to comment on Harinder Singh's case.

After the assassination, police searching Beant Singh's house reportedly found a photograph of Jarnail Singh Bhindranwale, the militant Sikh religious leader who was among hundreds of Sikhs killed in the Golden Temple raid, as well as some of his taped speeches, some extremist Sikh literature and a few foreign currency notes.

Known as a mild-mannered and withdrawn man, Beant Singh had earned a reputation for loyalty during his 12-year police career, including more than eight years of duty on Gandhi's security staff. In fact, he had been personally praised by the prime minister when a questioner once asked her if it was safe to keep Sikh security guards in view of the turmoil in the Punjab.

But after emptying his revolver on her from about a yard away on the morning of Oct. 31, Beant Singh repotedly threw down his weapon and shouted defiantly to her entourage: "I have done what I had to do. Now you do what you have to do."

Witnesses said Satwant Singh also dropped his weapon, and the two were hustled off by members of the Indo-Tibetan Border Police to a nearby guardroom, where they reportedly were held by about a dozen security men.

About 10 minutes later, shooting broke out inside the guardroom and both men were gunned down. Beant Singh died on the spot, but Satwant survived with multiple bullet wounds.

The police guards said later that the two men had made a dash for their captors' weapons. But this did not explain why, if they were so determined to escape, they had surrendered in the first place, or why it was necessary to riddle them with bullets when they were outnumbered and presumably could have been overpowered.

Little is known about Satwant Singh, who is said to have been recruited for the assassination by the older and more experienced Beant Singh.

The two were not related. Most male Sikhs by religious tradition share the surname Singh, which means "lion."

What has been established is that Satwant Singh joined the police in 1982 and was assigned to the prime minister's residence last year after some security training. He comes from the village of Agwan near the Pakistani border in the Indian Punjab district of Gurdaspur, which is known as a hotbed of Sikh militant activity. He reportedly had returned from leave in the Gurdaspur district shortly before the assassination.

Quoting intelligence sources, the Indian press has suggested that the assassins were directed or financed by Sikh militants in the Punjab or abroad. Among those singled out for scrutiny in the case has been Jagjit Singh Chauhan, a separatist leader based in London who publicly predicted that Gandhi would be killed in retaliation for the Golden Temple raid.

However, it also cannot be ruled out that Indian authorities may have an interest in promoting the theory of a wider conspiracy to discredit, and possibly extradite, extremist Sikhs who have been calling for an independent Sikh state of "Khalistan" in the Indian Punjab. Outside instigation or financial motives for the assassination may also be easier for Indian authorities to explain at home than a betrayal by trusted security guards for purely personal religious motives.

Since the assassination, government billboards and painted slogans have proliferated around the capital urging citizens to follow Indira Gandhi's ideals by placing "country above religion." In interviews at Sikh shrines in Pakistan and in the Indian Punjab, militant Sikhs have said that the religion demands vengeance for any desecration of its holiest shrine, the Golden Temple.

A separatist leader visiting Pakistan on a religious pilgrimage this month said it was "a religious duty of any Sikh to punish anyone who desecrates the Golden Temple." A Sikh temple administrator in Patiala in the Indian Punjab expressed similar sentiments and asserted that Beant Singh and Satwant Singh "are definitely admired here."