DALLAS -- The cases unfolded -- and unraveled -- one by one: drug bust after drug bust in which investigators later found little or no drugs in the evidence that was seized.

The FBI is examining the role of Dallas police and prosecutors in about 70 drug cases -- involving more than 40 defendants -- dismissed in recent months after lab tests revealed fake drugs or tiny amounts of the real thing mixed with large amounts of gypsum.

Whether it's a case of corruption or a massive system failure remains to be seen. But the botched cases have spilled over into the race for district attorney and angered the newly elected mayor. Some victims have threatened civil rights lawsuits, and Hispanic advocacy groups are calling for the resignations of the district attorney, city manager and police chief.

"It's the worst law-enforcement scandal I've heard of in the Northern District of Texas in the 20 years I've practiced here," said Paul Coggins, a former U.S. attorney.

Evidence tests

The scandal began unfolding last fall, when prosecutors notified Dallas police of evidence tests showing little or no illegal drugs. But defense attorneys say they had beseeched District Attorney Bill Hill to look into the cases long before that.

Hill defends his office, saying his prosecutors were first to discover the problem and send the evidence for lab tests. He said his staff found a pattern in some of the cases in early November and within two weeks halted all prosecutions involving two police officers and their informant, even though some of those cases may have been valid.

The Police Department placed the two narcotics detectives on administrative leave.

Police Chief Terrell Bolton has declined requests for interviews. The FBI also declined to give details, citing its investigation.

Glenn White, president of the Dallas Police Association, defended his suspended colleagues.

"They went out and did their job and they happened to have an informant that is unreliable," White said. "The informant realized if he delivered large amounts, he'd get more money."

Critics say police and county officials had several clues that the busts weren't legitimate. They question why Hill's office had not ordered lab tests before indicting defendants based on uncorroborated field tests by the arresting officers.

"How did these field tests, if they were actually run, mistake gypsum for cocaine?" Coggins asked.

White said most district attorneys and federal agents in Texas require lab tests before indicting suspects in large drug busts. Field tests are notoriously unreliable; something as innocuous as aspirin can test positive as cocaine, he said.

"This is not a corruption case," White said. "It's a system failed.