Two British tourists, who died after taking white heroin, have been named.
Shaun Brotherston, 20, and Bradley Price, 21, died after using the substance, which they had believed was cocaine.
The pair were on holiday in the Netherlands celebrating Mr Brotherston's 21st birthday.
According to local police, the men were found in a hotel room in Amsterdam, where traces of a white powder were found.
Police spokesman Rob van der Veen said that while a post-mortem examination is due to be conducted to confirm the cause of their deaths, preliminary tests on the white substance have confirmed that it was an opiate.
"We did a quick test to find out what kind of powder it was. It wasn't cocaine, it was an opiate - in this case, white heroin," Mr van der Veen said.
"We took the sample to the Dutch forensic institute and they found out that the contents of this material is the same contents as other material that we already seized in other cases where people got sick."
Investigators believe the men were sold the white heroin by the same street dealer implicated in the death of another British tourist last month. 14 other tourists were taken ill after snorting the substance, believing it to be cocaine.
Police had launched a public alert, warning British tourists of the drugs, with signs reading: "Extremely dangerous cocaine is sold to tourists."
Leaflets handed out at Amsterdam's Centraal railway station warn: "The last couple of weeks several tourists were hospitalized with severe health problems after using cocaine bought on the street. Research has revealed that the cocaine used was not cocaine but white heroin."
Tributes have been left on social media for the two best friends as their parents warned others to "learn from this tragic end."
Describing his son as "one in a million" Julian Price wrote on Facebook: "My heart is broken for you both, always together in life and now sadly in death, look after each other."
"Please all of you learn from his tragic end, love each other and make him and Shaun proud, at least they have each other in death as they did in life."