Finnish police said they have arrested two Iraqi brothers believed to have been members of the Islamic State group in Iraq and suspected of fatally shooting "11 unarmed and defenseless prisoners" in June 2014.

The National Bureau of Investigation said Thursday that the men arrived in Finland in September and were arrested Tuesday following an investigation with Finnish security police.

The agency said in a statement that IS posted a video online of the shooting of the prisoners in the city of Tikrit.

Police said both brothers were 23 but did not say whether they were twins, or whether the men were asylum-seekers. Some 17,000 Iraqis have sought asylum in Finland so far this year.

Later Thursday, police spokesman Jari Raty told reporters in a news conference that the suspected crimes were part of atrocities committed by IS in Camp Speicher outside Tikrit, where some 1,700 Iraqi soldiers were captured and then killed by IS militants in June last year.

He said the video posted by IS has provided details on the incident.

"The suspects are seen in the video together with other IS members shooting at tied-up, unarmed people on the ground," Raty said, adding that two men were unmasked in the situation.

Last month, Finnish officials said a 29-year-old Iraqi man was arrested on the suspicion committing war crimes in his home country in June 2014 but gave no further details.