Miami ? A federal judge in Miami on Wednesday acquitted the environmental group Greenpeace of illegally boarding a ship to bring attention to its cargo, 70 tons of mahogany cut from the Amazon rain forest.

U.S. District Judge Adalberto Jordan ruled that prosecutors had presented insufficient evidence to send the case to the jury. He granted Greenpeace lawyers’ request for a directed verdict of acquittal on the ship-boarding charge and a conspiracy count.

It was the first time a public interest group had been indicted for the protest activities of members. Activist groups across the country were watching the case because of its potential to chill political speech.

According to testimony, two Greenpeace activists used a rope ladder to climb aboard the Jade on April 12, 2002. They and four others were arrested, spent a night in jail, and paid fines. More than a year later, federal prosecutors indicted Greenpeace itself.

Greenpeace, a vocal critic of the Bush administration’s environmental policies, claimed the charges were brought to silence the organization and perhaps put it out of business. Had Greenpeace been convicted, its nonprofit status could have been in jeopardy.

Jordan did not base his ruling on the weighty political and constitutional issues raised by Greenpeace lawyers. Instead, he kept his decision simple, focusing on whether the government had proved Greenpeace violated an obscure 1872 law against boarding ships “about to arrive” at port.

The law originally was enacted to discourage brothels from luring sailors from ships. The practice was known at the time as “sailor mongering.”

Jordan found that the Jade, a 965-foot cargo ship, was about six nautical miles from the Port of Miami when two Greenpeace activists climbed aboard and attempted to unfurl a 40-foot banner: “President Bush: Stop Illegal Logging.”

Jordan ruled the ship was too far out to sea to meet the definition of “about to arrive” at port. He added that the Jade’s location was “fortuitous” for Greenpeace.

“Greenpeace is now on notice that this statute is out there,” Jordan said.

Prosecutors Cameron Elliot and Thomas Watts-FitzGerald declined comment. The government cannot appeal Jordan’s decision.

Later, the U.S. Attorney’s Office said in a statement that while respecting the judge’s decision, the office “remains undeterred in prosecuting those persons who illegally attempt to board ships at the Port of Miami or otherwise threaten port security.”