Brno Police spokeswoman Petra Vedrova identified the man as Kevin Dahlgren, born in 1992, and released his photograph. A Facebook page showing the same photograph says that Dahlgren is from Palo Alto, California, and lives in Brno, the Czech Republic's second-largest city.
Local firefighters spokesman Jaroslav Mikoska said rescuers arrived to tackle a fire at a family house in the Brno neighborhood of Ivanovice late Wednesday and discovered "that a violent crime has likely been commited there."
He said firefighters immediately called police to the scene. He declined to provide more details, citing a police request.
The local CTK agency said two members of a local band -- Ukulele Orchestra jako Brno -- were among the victims. Dahlgren's Facebook page contained a link to the band, which announced Thursday that it has canceled all concerts for the coming days.
Vedrova said all four victims found in the house were related. Czech public television said they were a married couple and their two sons, and that Dahlgren was the nephew of the parents. Vedrova would not immediately confirm that.
Police asked the public for help finding Dahlgren, but warned that he is dangerous and likely has a gun. They did not immediately release any other details about the suspect or about the killings.
Brno is located 200 kilometers (125 miles) southeast of Prague. Police say they cooperated with international law enforcement agencies to confirm the man's identity.
Copyright 2013 The Associated Press.