The three-minute song, which played uninterrupted for nearly half an hour on both internet and FM stations, encourages Western ISIS supporters to join the fight in Iraq and Syria.

Mix Megapol only became aware of the issue after listeners in Malmo, Sweden’s third largest city, began to call the station.

Jakob Gravestam, the station’s marketing director, told news site 24Malmo that the incident has been reported to local authorities.

“It is of course very serious, especially as they played a song about recruitment for ISIS,” Gravestam said. “Someone has used a pirate transmitter to override our frequency and play this track.”

According to the Swedish Security Service, at least 300 people have left Sweden to fight for ISIS abroad over the past several years.

The Scandinavian country has repeatedly made headlines for its treatment of returning ISIS fighters.

In one such example, officials in the city of Lund argued that jihadists should be given free housing, a driver’s license and tax benefits upon their return.

“We cannot say because you made a wrong choice, you have no right to come back and live in our society,” Anna Sjöstrand, Lund’s municipal coordinator against violent extremism, said last year.

Terrorism experts fear the jihadist group will expand its propaganda efforts as it continues to lose significant territory in Iraq and Syria.