At least six suspected "masterminds" of the Mumbai attacks are still at large in Pakistan and there is mounting evidence of ISI's strong links to the 2008 strikes, an investigative report said on Monday.

"The evidence against at least half a dozen suspected masterminds of Mumbai who are still at large includes (David) Headley's statements implicating officers in Pakistan's ISI along with Lashkar-e-Tayyaba (LeT)," the report quoted officials as saying.

"There are also physical clues," wrote investigative reporter Sebastian Rotella in the report jointly published in The Washington Post and ProPublica.Com.

The Mumbai case could put Washington and Islamabad on a collision course, the report said, amid Pakistan's unwillingness to take firm action against those responsible for the attacks that killed 166 people, including six Americans.

US attorney general Eric H Holder Jr has vowed to prosecute the killings of the six Americans as required by the law.

The prosecutions of the Mumbai and Denmark plots are being led by US attorney Patrick J Fitzgerald in Chicago. But it is unlikely Pakistan would extradite the suspects to the United States, officials said.

And Pakistani courts tend not to convict accused radical Islamists, the report said. The FBI has identified a phone number that is believed to connect Sajid Mir, a suspected Mumbai attacks mastermind; David Headley and Pakistani intelligence officials.

Headley called Pakistani military officers at the number while working for LeT; the number was also called by an accused ISI spy who went on a secret mission with Mir in India in 2005, the report said citing investigators.

The Pakistani government denies any official link to the 2008 attacks. However, Rotella said that the question of Pakistani government's involvement drives a high-stakes debate.

Some Western anti-terrorism officials think that, at most, Pakistani officials provided limited state support for the Mumbai attacks.

"A senior US counter-terrorism official believes a few mid-level Pakistani officials had an inkling of the plot but that its dimensions surprised them. Others speculate that the government of President Asif Ali Zardari may even have been a secondary target because of his overtures to India and his opposition to extremism," the report said.

"Perhaps it was done by people who didn't like the way the ISI and the army were moving, particularly in Kashmir," a European official was quoted as saying. "May be it was a rogue operation destabilising the Pakistanis as well as the Indians," it said quoting the official.

In contrast, a number of Western and Indian anti-terrorism officials cite the in-depth scouting, amphibious landing and sophisticated communications used by militants during the Mumbai attacks as signs of Pakistan's involvement. "Headley's disclosures and Lashkar's history make it hard to believe that military leaders were unaware of the plan, they say," according to the report.

Rotella said Mir and Maj Iqbal are keys to the mystery because they allegedly connect LeT and the government.

Western and Indian investigators suspect that Mir is a former military or ISI officer, or at least had close links to the security forces. They believe that Maj Iqbal was an ISI officer using a code name.

A recent Interpol notice of an Indian arrest warrant gives only his rank and last name. "It remains to be seen whether Mir, Maj Iqbal and other suspected plotters will be successfully prosecuted. An Indian court convicted the lone surviving gunman (Ajmal Kasab) in June. But US officials say the Pakistani trial of the Lashkar military chief (Zakiur Rehman Lakhvi) and six lower-level suspects captured last year seems hopelessly stalled," the report said.

It noted that unlike Al Qaeda and other militant groups, Lashkar has not attacked the Pakistani government.

"But its professionalism, global networks and increasing focus on Western targets have made it one of the most dangerous forces in terrorism, many investigators say."

Recent warnings of Mumbai-style plots by Al Qaeda in Europe reflect Lashkar's influence in the convergence of militant groups that a British official calls "the jihadist soup in Pakistan," the report said.