A 28-year-old woman and a 19-year-old man were found dead at a hotel in Luleå, northern Sweden, late on Thursday.
The man was a resident of Luleå, but the two had checked into the hotel on Thursday. Just a few hours later, personnel found the pair dead in their room.
A number of yellow pills were found with the bodies, and police suspected they had ingested some sort of compound, regional paper Norrbottens Kuriren reported.
"It's hard to say if it's something new. What we were able to see was that it has very quick effects and must be very dangerous," Luleå policeman Lars-Ivar Johansson told the paper on Friday.
The unidentified pills have been sent to a criminal laboratory for analysis. Police expressed concern that similar pills may be doing rounds in Luleå.
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"When you buy pills, for example on the internet or through other sources, you can never be really sure what you're getting," Johansson said.