Roemer, who served as the 22nd US ambassador to India from July 2009 to June 2011, however did not divulge any further details of his conclusion that 26/11 could have led to a nuclear war between the two South Asian neighbours.
"Those attacks killed 177 people in Mumbai two years ago. Six Americans were killed. And they almost started a war between Pakistan and India that might have resulted in some kind of a nuclear war. So this self-radicalization issue is a critically important one," Roemer said yesterday at a discussion on Status of National Security and the Implementation of the 9/11 Commission's Recommendations.
The former US envoy, a member of the 9/11 National Commission on Terrorist Attacks Upon the United States set up in 2002, said he feared a scenario in which a terrorist group gets hold of nuclear weapon and an undetected self-radicalised or a cell in the United States.
"We're often asked a question, I think every one of us: What keeps you awake at night? And I would answer the question, two things: a terrorist group getting a nuclear weapon and a self-radicalized or a cell in the United States that is undetected and can pull off a catastrophic event here," Roemer said.
"Now, several months ago we had something that almost combined both. There was a person by the name of David Headley who was a terrorist living in Chicago who could travel between India, Pakistan and the United States seamlessly. And he was the guy that planned helped plan the attacks on Mumbai," he said in response to a question.
Roemer expressed concern that self-radicalized people can train themselves on the Internet, and the time frame was shrinking.
"Having spent the last couple of years abroad, I would say to now these self- radicalised people can train themselves on the Internet, and the five-year time frame is now down to sometimes months before they can be radicalised. It's really shrinking," he said.
Roemer referred to his conversation with a very high-level Indian official, a day after the al-Qaeda leader Osama bin Laden was killed in a secret raid in Pakistan's Abbottabad town.
According to the former US envoy, the Indian official pointed to the secret operation saying "this is precisely why the United States of America needs to be involved and active in the world, because you have the tenacity to keep going after people, the training of your joint operations to do it better than anybody else and the moral courage to do it the right way", he said.
Roemer underlined that successive US administrations has been able to degrade the al Qaeda leadership over the years.
"So with that successful strike, I think it shows that the United States and successive administrations, the Bush administration and the Obama administration, have done things to degrade the al Qaeda leadership, that have helped put the US in a stronger position," he said.