AMSTERDAM (AP) — Authorities in Amsterdam warned Friday that potentially lethal white heroin is being sold as cocaine in the city after three Danish tourists became ill this week.

Three people died and 17 became seriously ill with breathing problems late last year after inhaling heroin in Amsterdam, a city whose reputation as a center of free-wheeling drug culture draws tourists from around the world.

City officials said a major public awareness campaign linked to those earlier cases will be resumed.

The campaign, which targets tourists, involves large signs around the city warning in flashing lights that "Extremely dangerous cocaine is sold to tourists" in the area, as well as posters and flyers in bars, pot-selling cafes and hotels.

Police launched a major investigation into the heroin sales last year, but never arrested a suspect. Police are now investigating whether the heroin sold to the Danish tourists came from the same batch that caused the deaths and injuries last year.

They have also released grainy photos taken by a security camera of a man suspected of selling the heroin to the Danes.