Offering an unprecedented level of cooperation to Indian authorities probing the Mumbai terror case, the FBI has said it will not seek custody of the perpetrators of the ghastly attacks, which also killed six Americans, and will let India prosecute them.

Under federal laws, FBI, the primary investigative arm of the US Government, needs to investigate and charge sheet and bring the perpetrators to justice whenever an American citizen is killed overseas.

"This is the major tragedy in India, so it is fully appropriate that Indian authorities handle the investigation and prosecution," a FBI spokesperson said.

"If at some point many many years down the road... there is an opportunity for the United States to prosecute them for the killing of Americans, our prosecutors would take a look at that point," he said.

"There is no stature limitation on murders, so it does not matter when that time would be but absolutely the priority here is let India to handle the investigation and prosecution," he said.

At this point of time FBI is not registering a separate case against those responsible for the killing of US citizens in the Nov 26 Mumbai attacks. Instead, it is extending all help to India in investigating the case.

"The FBI will work with the Indian authorities as well as our partners to cover all leads, wherever they may take us," the spokesperson said.