When the police found marijuana on Justin Bieber's tour bus on Sunday during a search as it crossed into Detroit from Windsor, Canada, the singer was not found on the bus at the time, according to Kenneth Hammond, chief officer and public affairs liaison for U.S. Customs and Border Protection in Detroit.

The Canadian pop musician performed Sunday night at Joe Louis Arena.

In an e-mail Monday night, Hammond said that the bus was trying to enter the U.S. via the Ambassador Bridge. A drug-sniffing dog forewarned authorities to the presence of drugs. Custom officers found and held a small amount of marijuana and drug paraphernalia, Hammond said. They issued the driver a civil penalty and the bus and its passengers were allowed to continue on. No further details were available.

The U.S. Customs and Border Protection said in a statement, "The tour bus, associated with musician Justin Bieber, was en route to a concert scheduled later Sunday night in Detroit. During the secondary inspection, a K-9 alerted to the presence of drugs and a subsequent search of the bus revealed marijuana and drug paraphernalia. Officers seized the marijuana and paraphernalia and issued a penalty to the driver. The bus and passengers were released without further incident."

This is the second time the police have found illegal drugs on the singer's bus. In April, Swedish police discovered drugs and a stun gun on the pop star's bus after reportedly smelling marijuana coming from inside.

"The police went onto the bus and searched it and found a small amount of narcotics," police spokesman Kjell Lindgren said. "We don't know who had the drugs or who smoked them, so it will be hard to link them with any individual."

While the singer never admitted using illegal drugs, rumors have linked him to marijuana. Shorly after the photos came out of the internet, he wrote this: "i see all of u. i hear all of u. I never want any of you down. i love u. and..thank u."

Back in March, authorities found illegal substance from a passenger on a Rihanna tour bus.