Greek authorities arrested two aid workers and are probing another 30 from the same group on charges of unlawfully smuggling refugees and migrants into the country, as human traffickers do frequently from Turkey, sending them to nearby Greek islands.

Other charges included spying and laundering money, police said, according to the news agency Reuters and other media reports.

The suspects, 24 foreigners and six Greeks, had been active on the island since at least late 2015 as part of a non-governmental organization, which Agence France Presse (AFP) identified as the  Emergency Response Centre International (ERCI).

Lesbos, not far from Turkey in the northeast Aegean Sea, is the prime destination for refugees and migrants fleeing civil war and strife and who go to Turkey as a jumping off point to get into the European Union through Greece, the closest entry point.

There are more than 15,000 on the islands and another 49,000 on the Greece mainland, with more arriving weekly during a suspended EU swap deal with Turkey. The BBC, which got into a detention center on Lesbos, said it was the worst refugee camp in the world.

More than 19,000 refugees and migrants have crossed to Greece from Turkey so far this year, according to data from the United Nations refugee agency, UNHCR. Police said the suspected crime ring allegedly “provided direct assistance to organized migrant trafficking rings.”

“The activities of an organized criminal network that systematically facilitated the illegal entry of foreigners were fully exposed,” a police statement said.

Members of the group were in contact with migrants on social media groups and “actively assisted” their illegal entry into Greece from 2015 onwards, according to the statement.

The accused were also said to have tracked incoming refugees and migrants by unlawfully monitoring Greek Coast Guard and EU border agency Frontex radio traffic, authorities said.

Among those arrested was Sara Mardini, a 23-year-old Syrian refugee student and scholarship recipient of Bard College in Berlin, said AFP.

Alongside her sister Yusra – a goodwill ambassador for the UNHCR, which is the Unitaed Nation’s refugee agency – she took part on the refugee team at the Rio 2016 Olympics.

A year earlier, the sisters had employed their swimming skills to pull the waterlogged boat that brought them over from Turkey with another 18 people onboard.

“They did nothing wrong, they want to help people out of idealism,” said Yusra’s swimming coach Sven Spannekrebs.

“I was on Lesbos a few weeks ago and saw what all the volunteers are doing: A great job,” he told AFP via email.

Spannekrebs said Mardini and another detainee, Sean Binder, were “long term volunteers for ERCI (and) never engaged in any illegal activity as suggested by the authorities,” and that the radio channels they were accused of monitoring can be heard by anyone.

Police said two ERCI activists had already been arrested in February onboard a jeep with concealed fake army license plates, the news agency also said.

Violence has often flared in the detention centers and camps on the islands with people waiting up to two years and more for their asylum applications to be reviewed. They are stuck in Greece with no where else to go because the EU closed its borders to them and reneged on promises to help take an overload from Greece and accept refugees and migrants in other countries.