Newly unsealed criminal complaints also allege the defendants sought to have the group removed from a list of terrorist organizations and to obtain classified intelligence by bribing U.S. officials.
The eight defendants, all men, were involved in "the procurement of military equipment, communications devices and other technology, fundraising and money laundering through charitable organizations and a myriad of other criminal activity, including conspiracy to bribe public officials," according to the court papers released Monday.
At a hearing in federal court, six defendants, all of Sri Lankan descent, were ordered held without bail on charges of conspiring to provide material support to a terrorist organization. If convicted, each could face up to 15 years in prison. Two other defendants were held elsewhere.
A defense attorney confirmed Nachimuthu Socrates, described in court papers as a Tamil Tigers supporter based in North America, had been arrested in Connecticut.
"We plan to fully and vigorously contest the charges," said his lawyer, Gerald Del Piano.
Socrates, an engineer who was born in India and came to the U.S. in 1976, was arrested around early Monday, a family member said.
Authorities said he made interim bribery payments connected to the schemes. They said he tried to buy a classified intelligence document that referred to a U.S. investigation into a charity suspected of being a front for a Tigers fundraising organization.
The U.S. Department of State added the Tamil Tigers, which began fighting in 1983 for a separate state in Sri Lanka's northeast, to its list of foreign terrorist organizations in 1997. The designation bars the group from raising money, obtaining weaponry or lobbying for support in the United States.
In a police operation, undercover agents posing as Department of State officials were offered millions of dollars (euros) during a series of secret meetings in a New York apartment, one of the complaints said.
At a meeting in July 2005, Socrates asked undercover agents whether they "could stop the United States government from sending arms to the Sri Lankan government" and "provide intelligence about this issue," the papers said.
Another defendant "made it clear that he had traveled to the United States on behalf of (the Tamil Tigers') senior leadership in Sri Lanka," including the group's evasive founder, Velupillai Prabhakaran, the complaint said.
The undercover agents told the defendants that to be pulled off the terror list the group "would have to renounce suicide bombings and the use of children as soldiers," the court papers said.
Other defendants were recorded in a separate operation last month negotiating for the purchase of 10 Russian-made missiles and 500 AK-47 rifles, prosecutors said.
"We need something for Kfir," one defendant said, according to court papers, referring to the type of fighter jets used by the Sri Lankan air force to strike rebel strongholds.
A recent surge in fighting has threatened to drag Sri Lanka back into civil war, four years after the Tamil Tigers and government reached a cease-fire brokered by Norway.
The rebels fought for almost two decades to carve out a separate homeland for Sri Lanka's 3.2 million ethnic Tamil minority. The civil war killed about 65,000 people.
Hundreds of people, many of them civilians, have been killed in renewed fighting since late last year.