"Today's verdicts illustrate the department's successful efforts to hold accountable bank executives responsible for this global fraud scheme," Leslie Caldwell, head of the Justice Department's criminal division, said in a statement.

Allen, 44, slumped over a table in court as the jury foreman read the verdict, while Conti, 46, held his head up. Lawyers for both men said they planned to appeal.

"This is just round one," Michael Schachter, Allen's lawyer, said. "Tony Allen looks forward to pursuing all available options. He is disappointed with the verdict."

GLOBAL INVESTIGATION

Libor, or the London interbank offered rate, is a short-term rate financial institutions charge each other for loans that is calculated based on submissions by a panel of banks. Hundreds of trillions of dollars in short-term interest rates, swaps and other financial products are pegged to Libor.

The case was the first by the Justice Department to go to trial following global investigations into whether banks submitted artificial rate estimates to bolster profits on trading derivatives tied to Libor.

Those investigations resulted in charges against 22 people in the United States and United Kingdom and around $9 billion in regulatory settlements with financial institutions.

Their trial follows an earlier one in London involving yen Libor manipulation that led to the conviction of Tom Hayes, a former UBS AG and Citigroup Inc trader who was sentenced in August to 14 years in prison.

Another trial is meanwhile ongoing in London for six former brokers accused of manipulating yen Libor rates. They have pleaded not guilty.

Among the 13 people charged by the Justice Department were seven former Rabobank traders, including Allen and Conti, who earlier this year waived their right to extradition to fight the charges.

Prosecutors said Allen, 44, and Conti, 46, participated in a five-year conspiracy at Rabobank to rig U.S. dollar and yen Libor rates in order to gain an unfair advantage trading derivatives linked to the rate.

Prosecutors relied on testimony by three former Rabobank traders who pleaded guilty as part of cooperation deals, as well as emails and instant messages they sent at the time.

Libor was supposed to be an "impartial market tool," Paul Robson, one of the traders, testified.

"But we were biasing submissions or shading it to help the traders," he said.

Lawyers for Allen and Conti countered that while others at the bank may have been trying to rig Libor, their clients had submitted honest rate estimates.

They argued prosecutors took documents out of context, and that cooperating witnesses lied about their clients' role in the scheme in hopes of getting lenient sentences.

Allen opted to testify in his own defence, telling jurors he rarely was involved personally in submitting numbers used to calculate Libor and that he in those few instances ignored traders' requests to bias the rate.

'CAN'T BE TOO OBVIOUS'

The jury of six men and six women included an architect, a nurse practitioner, and a T-Mobile sales representative, and delivered its verdict on the second full-day of deliberations in the fourth week of trial.

Howard Wasserfall, a retired letter carrier on the jury from Manhattan, said jurors concluded the evidence showed they attempted to discreetly manipulate Libor.

"If you have a scheme, the only way the scheme can work is that it can't be too obvious," he told reporters outside of court.

Post-verdict, Robson's testimony is expected to be the basis of a hearing before U.S. District Judge Jed Rakoff over whether the case should be dismissed on the grounds that the defendants' rights against self-incrimination were violated.

The defendants' contend the case was tainted as Robson had before agreeing to cooperate with U.S. investigators reviewed Allen and Conti's compelled 2013 testimony to the U.K.'s Financial Conduct Authority in a related probe.

As a result, any information Robson provided U.S. authorities may have been influenced by reviewing Allen and Conti's testimony, their lawyers contend. The Justice Department rejects that contention.

Beyond Allen, Conti and the three cooperators, U.S. prosecutors have also brought charges against two other Rabobank traders including Paul Thompson, who is facing extradition after being arrested earlier this month in Australia.

The case is U.S. v. Allen, U.S. District Court, Southern District of New York, No. 14-cr-272.

(Reporting by Nate Raymond in New York; Editing by Alden Bentley)

By Nate Raymond and Brendan Pierson