Seven men accused of brutally murdering Costa Rican environmental activist Jairo Mora and raping four American and Spanish colleagues went on trial Monday after their initial acquittal was overturned.

Mora, a 26-year-old conservationist working to protect sea turtle nests, was beaten unconscious, tied to a pick-up truck and dragged along a Caribbean beach until he suffocated in the sand in May 2013.

Four female volunteers who were working with him -- three Americans and a Spaniard -- were tied up, held for hours and raped by the assailants.

The crime harmed the Central American country's reputation as an eco-tourism paradise and drew international condemnation.

Prosecutors claim the defendants were part of a band who illegally trafficked turtle eggs and saw Mora as a menace to their business.

The eggs, considered a delicacy by some, fetch around $1 each on the black market.

But judges at the original trial criticized the prosecution for botching their case and acquitted the seven men a year ago.

An appeals court overturned that ruling and ordered a retrial, which is expected to run for about two months.

The defendants face up to 50 years in prison if convicted.

Mora monitored nesting turtles, recorded the number of nests and tried to rescue eggs. His program was entirely volunteer-based and funded by donations.

He had complained to journalists that the area's environmentalists were threatened by criminal organizations, apparently because poachers were linked to drug traffickers.