Swedish police have opened a hate-crime investigation after swastikas were spray-painted on the entrance of a mosque in downtown Stockholm.

Omar Mustafa, the head of Sweden's Islamic federation, says employees discovered the vandalism as they arrived to open the mosque Thursday morning. He posted pictures of the graffiti on Twitter.

Mustafa said the mosque is targeted by hate mail or vandalism about twice a month, but this is the first time the entrance was defaced with swastikas since the mosque was built in 2000.

Stockholm police said its hate-crimes unit was involved in the investigation, but had no suspects Thursday.

Statistics from the Swedish National Council for Crime Prevention shows about 5,500 hate crimes were reported to police in 2012. Six percent targeted Sweden's Muslim minority.