Two British tourists have died in Amsterdam after reportedly snorting white heroin which police believe they mistook for cocaine.
Their bodies were found in a hotel room in the south of the city on Tuesday next to traces of white powder.
Police in the Netherlands say the men, who were aged 20 and 21, were from England.
Officers told Newsbeat they believe the same dealers were involved as in the death of another British man last month.
Police spokesman Rob van der Veen says since mid-September, 17 people have needed medical attention after taking white heroin in the city. All of them were tourists.
The 14 who survived received medical attention quickly - many of them needed CPR.
Mr van der Veen told Newsbeat: "These are the first cases the police have seen where white heroin has been taken as cocaine in the Netherlands."
A post-mortem examination will be carried out later this week to determine exactly how the two men died, but he says the police have "strong evidence" it was from snorting the drug.
"We checked the powder and it looked like cocaine, but after testing it we found it was white heroin," he says.
"The contents were the same as material we seized in other cases where people got ill."
So what is white heroin?
"It is a sort of luxury drug," says Mr van der Veen, "people with a lot of money sometimes buy it, but normally they use it in another way.
"It looks like cocaine, but when people snort it the result is respiratory failure.
"Brown heroin is much more common," he says.
Mr van de Veen says white heroin is "much more expensive" than cocaine, but the dealer "sold it for the same price [as cocaine] several times".
"We think he doesn't know what he is selling", he says.
Officers think it's only tourists who have been affected because the dealer, or ring of dealers, sell drugs on the streets, and "speak very good English".
The police are calling for tourists to stay away from drug dealers.
Since October they have distributed warning leaflets, particularly aimed at British tourists at hotels and hostels, and in tourist areas.
The leaflets say: "[In] the last couple of weeks several tourists [have been] hospitalised with severe health problems after using cocaine bought on the street."
Mr van der Veen says: "Of course we know that the young people don't all come to Amsterdam to see the tulips, but they have to very aware of what they take.
"If they go to a coffee shop and use some weed then that would not be a big problem, but when they start taking ecstasy or cocaine they have to be aware of the danger because you cant see the difference between cocaine and white heroin.
"If you look at ecstasy some people aren't aware of how strong it sometimes is and they can get into trouble by taking too many pills," he adds.
The police have yet to release the names of the two men who died on Tuesday.
The Foreign Office says it's providing assistance to family members.