Little is known about the terrorist group that calls itself Deccan Mujahideen and claimed responsibility for the series of attacks Wednesday, in Mumbai, India, that left more than 150 dead, including as many as six Americans.

So far, most security analysts, intelligence experts and news reports have focused on two potential culprits: Lashkar-e-Taiba, a terrorist group in Pakistan, and Indian Mujahideen, a shadowy organization in India.

Lashkar is the leading contender, a U.S. counterterrorism official told FOX News, though the group denied any role in the attacks.

"Lashkar-e-Taiba strongly condemns the series of attacks in Mumbai ... Lashkar has no association with any Indian militant group," Abdullah Gaznavia, chief spokesman of the group, told Reuters.

Since Lashkar-e-Taiba was founded in 1989 in the Kunar province of Afghanistan, the group has grown into one of the largest and most active terrorist organizations in South Asia, and its fighters have been actively fighting Indian security forces in Kashmir since 1993.

India has accused the group of carrying out deadly explosions in Mumbai in 2003 that killed 55 people and injured 180, as well as an armed raid on India's parliament in 2001 that brought India and Pakistan to the brink of all-out war.

But the group has also been accused of carrying out attacks in Pakistan in opposition to the policies of former President Pervez Musharraf, leading to its ban in Pakistan since 2002.

The group is known to operate training camps that have been tolerated by Pakistan's powerful Inter-Services Intelligence (ISI) agency because of their potential use in conflicts against India and in Afghanistan. Lashkar-e-Taiba and Al Qaeda have shared training camps in Pakistan, but no one yet is linking Al Qaeda to the attacks in Mumbai.

A Hindu newspaper reported early Saturday that three of the captured terrorists have confessed to being members of Lashkar-e-Taiba, with one of the suspects allegedly from Pakistan's Peshawar area, and local media report that one of the captured gunmen was identified as Abu Ismail from Faridkot, Pakistan.

And police reportedly intercepted communications between the terrorists in the Punjab language, which is spoken in Pakistan.

But some believe the group could be related to Indian Mujahideen, which claimed responsibility for serial blasts in Delhi in September that killed 20 people and bombings in Ahmedabad in July, when 45 died. In September, the group warned Indian authorities that Mumbai would be the next target.

India officials believe Pakistanis are behind the attacks.

"According to preliminary information, some elements in Pakistan are responsible for Mumbai terror attacks," Pranab Mukherjee, India's foreign minister, told reporters on Friday in the western city of Jodpur, though he declined to identify the evidence. He added that Pakistan had assured New Delhi it would not allow its territory to be used for attacks against India.

India has long accused Islamabad of allowing militant Muslim groups, particularly those fighting in the disputed Himalayan region of Kashmir, to train and take shelter in Pakistan.

However, with attention focusing on groups with links to Pakistan and the Kashmir region, Husain Haqqani, Pakistan's ambassador to the United States, issued a strong response.

"It is unfair to blame Pakistan or Pakistanis for these acts of terrorism even before an investigation is undertaken," he said. "Instead of scoring political points at the expense of a neighboring country that is itself a victim of terrorism, it is time for India's leaders to work together with Pakistan's elected leaders in putting up a joint front against terrorism."

And earlier Friday, Pakistan's Defense Minister Ahmed Mukhtar strongly affirmed his country "is not involved in these gory incidents."

Meanwhile, a team of FBI agents is flying to India to investigate, and a second group of investigators was on alert to join them if necessary, U.S. authorities said, speaking on the condition of anonymity because of the sensitivity of the operation.

FOX News' Catherine Herridge and The Associated Press contributed to this report.