Did British special forces have a role in 'Operation Blue Star', the operation launched by the Indira Gandhi government in 1984 to flush out militants in Amritsar's Golden Temple, the holy of holies for Sikhs?
The probe launched by UK Prime Minister David Cameron into the issue is expected to answer this question today.
Cameron launched the probe last month after documents disclosed under the UK's 30-year secrecy rule suggested a British officer helped the Indian authorities with plans to remove Sikh separatists from Harmandir Sahib, more generally known as the Golden Temple.
Sikh groups in the UK on Monday were quick to express reservations about the ''limited scope'' of the investigation. In a letter to the prime minister, the chairman of the Sikh Federation Bhai Amrik Singh said he was "hugely disappointed" with the inquiry's "narrow terms".
"We are dismayed the terms of the review were only formally made available almost three weeks after the review was announced and only days before an announcement of the results of the review are expected in Parliament," he said.
Foreign Secretary William Hague is due to make a statement to parliament today on the issue.
Adding fuel to the fire of India's opposition parties, the documents reportedly show that the then UK Prime Minister Margaret Thatcher agreed to provide British support for the operation, and an officer of the elite Special Air Service (SAS) travelled to India and drew up a plan which was approved by Indira Gandhi.
It is not known how close the February 1984 plan was to the eventual deadly Operation Blue Star raid, which not only left 500 dead but triggered a cycle of bloody revenge attacks.
It is not known whether the plan referred to in the documents from February 1984 was used by the Indian government. Retired Lieutenant-General K S Brar, who led the final June 1984 assault, said this was the first he had heard of the UK participation.
Two letters released from the archives, both marked "top secret and personal", reveal details about the SAS advice.
One of the documents, a letter from foreign secretary Geoffrey Howe's private secretary to his counterpart in the home office interior ministry, warned that the operation could trigger tensions in Britain's Indian community, "particularly if knowledge of the SAS involvement were to become public".
Cameron had asked cabinet secretary Jeremy Heywood, Britain's most senior civil servant, to investigate the new information.