The first trial in the US of a member of Colombia's largest left-wing rebel group, Farc, has begun in Washington.
Palmera has denied any role in kidnapping Americans
Ricardo Palmera is accused of plotting to kidnap three US citizens after their plane crashed in Colombia.
Prosecutors say Mr Palmera tried to use the hostages to get Colombia to release rebels, while the defence says he is just a "pawn in a power play".
Mr Palmera was captured in Ecuador and later extradited to the US in late 2004 by Colombia.
He has already been convicted by a Colombian court of kidnapping and rebellion and sentenced to 35 years in jail.
But BBC correspondents say the US prosecution is being seen as a trial of the whole of the Farc, which is considered a terrorist organisation by the US government.
The main charges in the US trial facing Mr Palmera - better known as Simon Trinidad - relate to the crash of a US plane in Colombia in February 2003.
Three US government contractors were captured by the Farc when their aircraft crashed during a mission to find illegal drug crops.
The Americans - Thomas Howes, Keith Stansell and Marc Gonsalves - are still thought to be held by rebels in south-eastern Colombia.
Their relatives were in court for the Washington trial.
Prosecutor John Crabb said Mr Palmera was guilty of hostage-taking, conspiracy and providing support to terrorists.
"We'll trade you hostages - including three Americans - for all Farc prisoners in jail. That was Simon Trinidad's offer," Mr Crabb said.
"He admitted he was a member of Farc; he admitted Farc takes hostages."
But the defence said Mr Palmera, 56, was only a middleman trying to negotiate a deal between rebels and the Colombian government.
"This man had nothing to do with the capture of the three Americans," his lawyer, Robert Tucker, said. "He became the pawn in the power play."
Farc has been fighting the government for more than four decades.