Washington:
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.
"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-Taiba (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.