Lars Bystrom, spokesman for the Stockholm police, told The Associated Press a small amount of drugs and a stun gun were discovered during a search of the bus, which had been parked under the Globen concert venue in Stockholm, where Bieber was performing Wednesday.
Bystrom declined to identify the drug, saying that it was sent to a lab for analysis. He said no one was arrested since the bus was empty at the time of the search.
Swedish police decided to act after smelling marijuana coming from inside the bus while it was parked outside the hotel where Bieber was staying in the Swedish capital. The drug squad searched the bus during the concert, Bystrom said.
The incident is the latest in Bieber's tumultuous European tour, which has included a monkey detention, a Holocaust museum furor and a health scare.
In Britain, the 19-year-old singer struggled with his breathing and fainted backstage at a London show. He was taken to a hospital, only to be caught on camera clashing with paparazzi.
In Germany, the Canadian teenage idol had to leave his monkey in quarantine since he didn't have the necessary papers for the animal.
In the Netherlands, Bieber became the focus of intense criticism for writing an entry into a guestbook at the Anne Frank House museum in Amsterdam, saying he hoped the Jewish teenager, who died in a Nazi concentration camp, "would have been a Belieber" -- or fan of his -- if her fate had turned out differently.
The comment provoked a flood of comments on the museum's Facebook page, with many people criticizing the singer for gross insensitivity.
Anne Frank hid with her family in a small apartment above a warehouse during the Nazi occupation of World War II. Her family was caught and deported, and Anne died of typhus in Bergen-Belsen in 1945. She was 13 years old when she began keeping her diary in 1942. Like many teenage girls, she made a collage of the celebrities of her day -- movie stars, dancers, and royalty -- and kept it on her bedroom wall.
In Norway, where Bieber enjoys enormous popularity, education officials in a remote district rescheduled mid-term exams for high school students so that the singer's fans could attend the concert in the capital and not have to worry about missing the tests.
By Thursday afternoon, Bieber had left Sweden and was in Finland, where he is due to play in the capital Helsinki on Friday evening.
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