Sea Shepherd founder Paul Watson skipped bail and fled Germany over fears Japan would seek his extradition if he was sent to Costa Rica, the organization suggested Thursday.
The head of the marine conservation group left Germany "for an unspecified destination", his lawyer told a Frankfurt court on Wednesday, prompting an arrest warrant to be reissued.
Earlier this year Costa Rica filed an extradition request on charges stemming from a high-seas confrontation between a Sea Shepherd ship and a Costa Rican vessel over alleged illegal shark finning in 2002.
Mr. Watson, a Canadian, whose organization is known for its aggressive attacks on Japanese whalers, was accused of "putting a ship's crew in danger".
The 61-year-old was arrested at Frankfurt airport in May and detained for a week before being released on bail after paying €250,000 ($303,500) and being ordered to appear before police twice a day.
Susan Hartland, Sea Shepherd's administrative director, confirmed that he had fled, but said she did not know where.
"We have reason to believe from a reliable source that, once in Costa Rica, the Japanese government may have sought extradition of Captain Watson to Japan to answer charges related to obstructing their illegal whaling activities," she said.
"We have no further information and are not in touch with him. We will do our best to provide more details as we learn more."
Mr. Watson, who was tweeting as recently as Tuesday but made no mention of where he was or what he planned to do, has previously suggested that Japan might be "putting pressure" on Germany to carry out the extradition order.
Sea Shepherd is best known for its annual pursuit of the Japanese whaling fleet in Antarctica, using increasingly militant methods to halt the hunt, including the boarding of vessels.
This year, after setting off from Australia, the group hurled stink bombs at the boats on the high seas and used ropes to try to tangle their propellers in a series of exchanges which saw the whalers retaliate with water cannon.
The whaling fleet killed less than a third of the animals it planned to because of the sabotage attempts.