Those sex sting arrests in Murphy, Texas, were headlines on NBC's To Catch a Predator but the arrests may have been unlawful and many may never go to court.

News 8 broke the story on the problems with the cases.

Here we have an exclusive interview with Collin County District Attorney, John Roach, on why the cases are weak.

Last year, Collin County prosecuted more than 11,000 cases - evidence that District Attorney John Roach is not soft on crime.

But when it came to prosecuting 23 men arrested by Murphy police in a sex sting with NBC news and an internet watchdog group, Roach said 'no.'

"We have questions about whether the arrests were even legal," he said.

One reason - Murphy police had no arrest warrants.

"What you saw on television wasn't the crime. The crime had already been committed when the guys showed up at the house."

The men showed up after allegedly soliciting sex on the internet and on the phone with members of Perverted Justice who were posing as minors.

But in 15 of the cases, no part of the crime took place in Collin County. It was on computers literally all over the country.

The Collin County DA has no jurisdiction.

And he says, Perverted Justice's involvement creates more difficulties.

"In this case, Perverted Justice or the television network, were in control of this entire operation. And the Murphy Police Department was merely incidental and part of the program," said Roach.

News 8 has tried repeatedly to contact Perverted Justice. They have not responded.

They boast of a 100 percent success rate in bringing sexual predators to justice.

Their TV partner, NBC, declined to be interviewed for this story.

But Chris Hansen, who hosts To Catch a Predator for NBC says this about the show's record.

"Of the more than 200 men who've been arrested. None has gone free. They have either pleaded guilty or their cases are still pending," he writes in a statement.

But here are the statistics. Some 286 people have been arrested in Dateline/Perverted Justice stings around the country. Some 103 of them, 36 percent have pleaded guilty. More than 150 have yet to be tried. They're still on the streets.

Collin County warned the police department of potential problems before the operation.

The department went ahead on its own.

In a brief interview with News 8 two weeks ago, Murphy Chief Billy Myrick defended his actions.

Since then, Myrick has not returned our phone calls.

Only eight of the 23 cases show promise, Roach says, because of meager evidence and record keeping by Murphy police.

"If they had been in control of this situation we could have good cases to present. They're a professional organization and they know what they're doing. What they did instead though is abdicate completely to outsiders frankly who don't know diddly squat frankly about the law enforcement."

NBC pays Perverted Justice to run the stings. The DA has asked for the contract between the two but they won't provide it.

NBC's video tapes are key evidence in the case. The DA has asked for them without success.

"What is exactly the deal between the City of Murphy and NBC ? What is the deal between NBC and Perverted Justice? Who's getting paid what? Who has an axe to grind?" said Roach.

The throw-down arrests, hidden cameras, and confrontations make fascinating television for many but the DA says are unneeded for law enforcement.

"These guys weren't fleeing from the police. The probably would have gone home and there would have been plenty of time to make the cases and get the warrants and arrest them properly. But of course you don't get to see that part on television," Roach said.

E-mail bharris@wfaa.com.