A Houston police officer pulled Barry Demings over as he headed to work in Beaumont and plucked a spot of white powder off the floorboard of Demings' year-old Ford Explorer.

Demings had just detailed the SUV - and wondered later if a speck of soap upended his life.

"I never even saw it," he said, explaining how the officer dropped the speck into a small test kit and said "it came back for cocaine."

Demings was charged with felony drug possession based on the results of the primitive test that costs about $2 and has been found to have a high error rate. He was told he could face a sentence as long as 30 years based on old prior convictions - no one mentioned waiting for a crime lab to verify the officer's roadside result.

He insisted he was innocent but got scared and accepted a plea deal. He lost his job, his girlfriend and his Explorer. Upon release, he decided to leave Texas behind forever.

In 2015 - seven years later - the Harris County District Attorney's Office notified him that Houston's crime lab found no cocaine in the sample. He filed a writ of habeus corpus with the Texas Court of Criminal Appeals and was finally exonerated.

He is among 298 people convicted of drug possession even though crime lab tests later found no controlled substances in the samples, according to a far-reaching audit of drug cases by the Harris County District Attorney's Office. So far, 131 of them, like Demings, have had their convictions overturned in cases that go back to 2004. About 100 other cases remain under review for potential dismissal.

In all 298 cases, prosecutors accepted both felony and misdemeanor plea deals before lab tests were performed. The $2 roadside tests, which officers use to help establish probable cause for an arrest, cannot be used at trial as evidence under Texas law.

So far, only a few of the exonerees have been deemed "actually innocent" by the Texas Court of Criminal Appeals - a necessary step to win compensation from the state for being wrongfully convicted.

Strict detention policies

The New York Times and ProPublica reported last Sunday that over 200 wrongful drug convictions in Harris County had resulted from erroneous roadside tests. Beyond those cases, the Times and ProPublica estimated that at least 100,000 people plead guilty nationally each year to drug offenses based on the tests, which the Houston police and many other local departments continue using.

Other wrongful convictions identified in Harris County have come from officers' misidentification of additional illicit drugs, including prescription pills and marijuana, using means other than the roadside tests, according to a review of exonerations and court records.

In many of the wrongful convictions, defendants were jailed pretrial because they were unable to post bail - and accepted pleas in order to get out sooner, though some claimed innocence, according to court records and interviews. Many were sentenced to serve time in jail or prison.

State Sens. John Whitmire and Rodney Ellis, both Houston Democrats, said the wave of exonerations provides yet another compelling reason that Harris County judges should reform their tough pretrial detention policies. They argue that police should cite and release low-risk suspects accused of minor drug crimes until the evidence against them is verified by one of the county's accredited drug labs.

Avoiding 'tragedies'

Currently, even misdemeanor drug offenders with little or no criminal history are typically required to post bail. Those who cannot pay bail are more likely to plead guilty in order to get out of jail more quickly, local and national studies show.

"It's basic, common sense that if we're going to throw people behind bars for drug possession, we have to have a reliable and trustworthy means of determining their guilt," Ellis said."

"But regardless of the mechanism for testing drugs, a great number of these tragedies could have been avoided altogether if Harris County utilized alternatives to incarceration for low-level drug offenders."

The Harris County audit of drug possession convictions and related lab results going back to 2004 was launched in 2014 by Inger Chandler, an assistant district attorney in charge of the DA's conviction integrity unit, after a reporter from the Austin American-Statesman called her about reversals of several drug convictions by the Court of Criminal Appeals.

The following year, Harris County District Attorney Devon Anderson changed her policies and directed prosecutors generally to stop accepting guilty pleas in felony drug cases before receiving lab reports confirming the evidence. Plea deals are still accepted prior to lab testing in misdemeanor drug cases, and in some felony cases in which jailed defendants can qualify for probation.

The forensic evidence problems uncovered by Chandler's unit began around 2005, when Houston's city crime lab - then overseen by HPD - lost several staff members and simultaneously saw a huge increase in drug cases, which created a backlog.

Lab officials implemented a triage system for drug testing with the DA's office: Drug cases slated to go to trial would get processed first. For defendants who had accepted plea deals, the crime lab would later go back and test samples, often months or years after the guilty plea had been entered.

Chandler's audit of wrongful convictions has been possible because the Houston Forensic Science Center, formerly HPD's crime lab, preserved and tested the evidence even in the plea deal cases.

"We were keeping the evidence, and with the agreement with the District Attorney's Office that we would continue to process even if it was pled," said James Miller, manager of the center's controlled substances section. "Because we both understood there was always the possibility that the substance may not actually be illegal."

So far, prosecutors have identified and examined 456 flawed cases. Of those, 298 people had been convicted despite having no illegal controlled substances in their possession at all. In 29 of the 298 wrongful convictions, there had been no filing for relief because a defendant declined to pursue the case or faced other legal obstacles.

In other cases among the 456, the types or quantities of controlled substances were misidentified or there was too little evidence left to perform a confirmatory test.

About 78 percent of the 456 flawed cases came from the Houston Police Department, which still uses roadside tests that were developed in the 1970s. Chemicals in small vials turn colors when exposed to cocaine and other illegal drugs but can be easily misinterpreted by officers and can have high false positives, Miller and other experts said.

Stopping false arrests?

Mayor Sylvester Turner declined comment about the HPD's continued use of the tests, saying he had not been briefed about them.

Retired Houston Police Chief Charles McClelland has expressed concern about the tests' high rate of false positives but said they also can help prevent some flawed arrests.

"If you eliminate the field test completely …more people would go to jail," he said. "Because the officer would have no indication what the substance is, and you're going to be arrested. But if the field test is negative, you could have a pound of baking powder and be on your way."

Nicolas Hughes, an assistant public defender at the Harris County Public Defender's Office, so far has filed habeas corpus writs for more than 106 felony defendants before the Court of Criminal Appeals, which can formally exonerate them by vacating the judgments and remanding cases to the county, where charges are then dismissed.

Hughes argues that police who believe they find small quantities of illegal drugs on suspects could simply issue citations based on the field tests or other evidence but that formal charges should wait until a lab confirms the results.

If lab tests come back negative, he said, the citation could simply be dismissed without filing charges, saving jail and processing costs and preventing wrongful convictions.

Defense attorneys say the prosecutors' new policy of not accepting pleas in felony drug cases without confirmatory lab tests has had the effect of increasing pretrial jail time for people arrested for low-level drug offenses who can't afford bail and now must wait for lab results.

With this policy in place, the Houston and Harris County crime labs are working to prioritize pending cases and can produce results in as little as 10 days. Officials at the city's lab said they eliminated their drug test backlog in July 2015.

Prosecutors have continued to accept guilty pleas in misdemeanor drug cases because many of the accused can't post bail and are waiting in jail for adjudication.

Chandler said that many defendants charged with drug possession in misdemeanor court may be eligible to receive time served and be released from jail quickly once they take plea deals.

"Sometimes they would have ended up spending more time in jail than they would have if they didn't take the plea deal," she said. "So for right now, we didn't want to disrupt that process."

But at least 10 people who accepted misdemeanor guilty pleas in 2015 were exonerated in 2016 after lab results showed samples taken by police did not contain the illegal drug they'd been convicted of having, according to court records and summaries of exonerations posted by the University of Michigan Law School.

Long-term impact

Even a wrongful misdemeanor conviction can have a long-term impact on someone's job opportunities, driving record and reputation, said Alex Macias, who served as the court-appointed attorney for 19-year-old Carlos Matamoros, one of the most recent exonerees.

The teen had received deferred adjudication on a misdemeanor charge when a police officer arrested him after finding what the officer believed was a single Xanax pill - alprazolam -inside a car. Suddenly, prosecutors moved to revoke his probation and find him guilty in two cases. The teen, who could not afford bail, spent 10 days in Harris County jail mulling his options, but his "mom just wanted him back home - she wanted her boy out of jail," Macias said.

He pleaded guilty -and then lab results cleared him a few days later. Macias got the drug charge dismissed, but the other charge remains on the teen's permanent record.

James Pinkerton and Brian Rogers contributed to this report.