The Macedonian interior ministry said on Tuesday that it had arrested 20 members of a radical Islamic group suspected of killing five people near the capital Skopje in April.
Minister of the Interior Gordana Jankulovska said the perpetrators had fought in Afghanistan and Pakistan against the NATO-led international coalition.
Some 800 police took part in the operation to arrest the perpetrators. Most of the 20 people who were arrested are from Skopje and are charged with terrorism.
The ministry said that the operation, called "Monster", was still under way and that more information would be released in the next 24 hours.
On April 12, four men aged 18-20 and a 45-year-old man were found dead near Zeljezarsko Jezero, a popular angling location in Skopje's northern suburb of Radisevo. The murders sent tensions between Orthodox Macedonians and ethnic Albanians running high.
The inter-ethnic tensions, heightened by the killings that were committed on the eve of Easter, which Macedonians celebrate according to the Julian calendar, had started in February when a Macedonian police officer off duty killed two young Albanian men in the northwestern town of Gostivar. That triggered the worst violence since 2001, when the country came very close to a civil war.
In the first half of March, gangs of young men attacked buses in Skopje and other towns, injuring at least 15 people. The police arrested more than 30 suspects at the time, after which the violence started subsiding.
