A special investigation carried out over nine months by a research team from German public broadcaster ARD and several European media partners has uncovered evidence of brutal efforts to push back migrants along the border of Bosnia and Croatia.

According to the report, published Wednesday, special Croatian police units, called "intervention police" were involved in an illegal effort to force migrants back into Bosnia. 

Video evidence and police sources said the beatings, delivered mostly by police batons, were ordered by the Croatian government, in what has been dubbed by officials as "Operation Corridor."

How is the border operation carried out?

ARD, along with media partners including Germany's Der Spiegel, the French paper Liberation, Croatian outlets and Lighthouse Reports, which conceived the investigation, used drones to gather footage. Journalists used disguises, such as fishermen, to blend in and observe the beatings taking place.

Satellite imagery, hundreds of social media accounts, along with the usual reporters' work of speaking with sources, unveiled a systemic effort to forcibly halt the entry into the EU of refugees.

The police units usually hide their identity by wearing unmarked uniforms and balaclavas over their faces, according to Der Spiegel.

"When we find migrants in the forest or elsewhere, they usually lie down on the ground out of fear," a member of the border police told ARD.

"A policeman then walks along them and beats them on the legs with a baton," he said. 

The headquarters in Zagreb decides what to do with the migrants, whether to take them to the police station, push them back or start an asylum process, he added. 

What have the migrants said?

The journalists were also able to speak with the migrants in the forest after one incident in which Croatian intervention police had forcibly evicted a group trying to exercise their right to claim asylum.

The Pakistanis and Afghans they met were soaking wet and some were barefoot or wearing only socks. They showed off their injuries, which included long purple welts covering their backs, along with bruises and bleeding gashes on their upper arms and elbows.

A Doctors Without Borders (RSF) field coordinator Daniel Song said, "We can see from the injuries that people were hit 10 times or more. This was an act of violence."

One migrant told the reporters: "They took everything from me: Shoes, jacket, money, cell phone, everything I had. It was a bad situation, they hit everyone very hard. You can see, our backs, our arms."

The migrants said the policy were also verbally abusive: "They said, 'You are terrorists ... go to your country. You are not wanted here,'" one migrant said.

Evidence of EU border funding used in operation 

The team of reporters noted Operation Corridor was partly funded by the EU. Between 2014 and the present, approximately €177 million ($205 million) has been granted to Zagreb for "migration management".

The European Commission claims no knowledge of the operation and the beatings. But officials have reacted to the news.

The EU's Home Affairs Commissioner Ylva Johansson called the report "shocking" and said she was "extremely concerned" by it. She said she expected a thorough investigation into the matter. 

She added that the evidence presented in the report seemed credible and that people were systematically being subjected to violence on the EU border. She also said that the bloc's money was being misused to support forces that committed "unacceptable" acts.

The Commission asserts that in cases where EU-funded equipment was used to break the law, there could be penalties imposed or the funding could be halted.

Johansson will meet with interior ministers of Greece and Croatia, who are both in Brussels, later today.

The Croatian Interior Ministry also announced that it would investigate the incident by sending a team of experts to the location on the border. If it turns out that Croatian officials were involved, they will be held responsible, said a spokeswoman.