The victims, most of them Hispanic, ranged from a 16 year-old girl to a 46-year old man, and came from the city's northeast region.

Historically, it is normally a single pure drug that causes these kinds of deaths - a combination is new

Dr Joye Carter Medical examiner

By Wednesday, a mixture of cocaine and opiates - possibly heroin - had been found in 14 bodies.

"Historically, it is normally a single pure drug that causes these kinds of deaths. A combination is new," said Dr Joye Carter, the Harris County chief medical examiner.

No trace

The total number of deaths from suspicious causes related to drugs in Houston over the past few days is 18.

Tests on four of the victims have not so far revealed any traces of the mixture that apparently killed the others.

Authorities in Houston believe the spate of deaths may be related one particular batch of unusally pure or tainted heroin.

Houston police chief CO Bradford said the batch may now be used up or that drugs users may have been warned to avoid it.

Drug-related deaths are common in Houston

"It appears that the stuff is all used up now, or people are responding to the signal: 'Don't take it' ", he said.