The C-Star, the group wrote on Twitter, was “having technical difficulty. We’re resolving it. No distress. #DefendEurope.”
The group’s Italian leader, Lorenzo Fiato, did not respond to a request for a comment. But last month in Milan, where he lives with his parents and sleeps in a room surrounded by board games like “The Middle Ages,” he argued that the aid ships did not in fact rescue anyone.
“It’s not true the NGOs save lives,” he said, referring to the nongovernment organizations that operate many of the aid ships. “Yes, they rescue people from shipwrecks. But do they guarantee these people a new life?”
Mr. Fiato had spoken in heroic terms of his earlier effort to brave seasickness and stop — for a few minutes, anyway — an aid ship in Catania, Sicily, from leaving a port. For this voyage he had packed anti-seasickness gum and had a gleam in his eye when he spoke of his seafaring vessel.
“We have a 40-meter search-and-survey ship given to us by a private person who saw what we did in Catania,” he said, “and gave it to us for a cut-rate price of 60,000 euros.”
More than 95,000 migrants have arrived in Italy this year, and more than 2,000 have died at sea. The issue of migration, especially regarding the private aid ships that now rescue more than 40 percent of the migrants at sea, has dominated politics in Italy.
The suspicion that some aid groups have helped traffickers provided a chief motivation, and talking point, for the C-Star’s stated mission of monitoring the aid ships. But many humanitarian groups accused the C-Star of getting in the water to disrupt the saving of lives.
Mr. Fiato, echoing right-wing politicians, accused the aid ships of turning off their transponders so they could dodge the authorities as they colluded with human traffickers to bring migrants to Europe.
But Mr. Buschheuer of the rescue ship said that when his colleagues on the Sea Eye sought to identify the C-Star from afar, they could not see its name because its automatic identification system was not activated.
“It’s one of the things they accuse us of doing,” Mr. Buschheuer said.
Many aid ships dismiss the accusation that they are colluding with human traffickers as ludicrous. But in recent days, the Italian authorities boarded and confiscated a different German aid ship on suspicion of such collusion.
Titus Molkenbur, a representative for the confiscated German ship, denied the accusation and said in an interview last week that Italy was criminalizing rescue operations and that the seizure was a “politically motivated” retribution for his not signing a new code of conduct for nongovernment organization ships championed by the country’s tough interior minister, Marco Minniti.
A former Italian spymaster, Mr. Minniti has sought to strike accords with Libyans to stop human traffickers but has also demanded more monitoring of the aid ships.
As migrant landings in Italy have decreased, Mr. Minniti received key support this week from the influential Italian Bishops Conference, which warned humanitarian groups to obey the law and avoid having their idealism exploited into collaboration with traffickers.
But just as the political wind in Italy seemed to be starting to fill the C-Star’s sails, the eager crew of young white nationalists instead found themselves with engine trouble. It was a development Mr. Fiato perhaps foresaw as he prepared for his journey last month.
“More than smugglers shooting us,” Mr. Fiato said. “I’m worried about the unexpected.”