HOUSTON -- The U.S. saw a record number of exonerations in 2015, with nearly 40 percent of the cases involving individuals who were exonerated in homicides, a new report shows.

The National Registry of Exonerations said in its report Wednesday that 149 people falsely convicted of crimes were exonerated last year. That's 10 more than in 2014, the year with the previous highest total since the group began keeping records in 1989. The registry is a project of the University of Michigan Law School and has documented more than 1,730 such cases in the U.S.

Since 2011, the annual number of exonerations has more than doubled and there are now an average of nearly three exonerations a week, said Samuel Gross, a University of Michigan law professor and registry editor.

"What's driving it? Continuing increased interest and sensitivity and concern about the problem but also a focus on increasing activity by conviction integrity units," Gross said. The integrity units are divisions in various district attorney offices around the country that identify and correct false convictions.

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Texas, the second-most populous state, had the most exonerations with 54. New York, the fourth-most populous, was second with 17.

Homicides and sex crimes made up nearly half of all exonerations in the U.S. According to the registry, a record 58 defendants who were exonerated in 2015 had been convicted of homicide, with five having received death sentences and 19 having been sentenced to life in prison.

There were homicide exonerations in 25 states and the District of Columbia, with Illinois having the most (11 exonerations), followed by New York (9 exonerations) and Alaska (4 exonerations).

The registry's report also said there was a record 27 exonerations in 2015 for convictions based on false confessions, with 22 of those in homicide cases. Also, 44 of the 58 homicide case exonerations involved cases in which there was official misconduct by authorities.

"The thing that is most troubling to me about these cases is it's clear that for every innocent defendant who is convicted and later exonerated, there are several others who are convicted who are not exonerated because almost all the exonerations depend on a great extent on good fortune, on Lady Luck," Gross said.

Brooklyn District Attorney Ken Thompson, whose conviction integrity unit has had 17 exonerations in the last two years, said his unit has a clear philosophy: ensuring that justice is done.

Three of the exonerations Thompson's office helped obtain in 2015 were connected to a deadly 1980 fire that killed a mother and her five children. The three men who were convicted had their arson and murder convictions overturned in December after Thompson's office was able to cast doubt on the fire science that helped convict the men. The office also discovered the only eyewitness who had tied the men to the fire recanted the identification on her death bed. One of the men whose conviction was overturned had died in prison in 1989.

"This case is indicative of our determination here in Brooklyn to get to the bottom of our cases, to get to the truth," Thompson said.

For the second year in a row, the large number of exonerations in Texas was due in part to individuals who had their drug convictions dismissed after lab tests determined they never had illegal substances. In 2015, there were 42 drug case exonerations in Harris County, where Houston is located - up from 31 in 2014. In these cases, individuals pleaded guilty before a lab test was completed.

Inger Chandler, chief of the conviction review section with the Harris County District Attorney's Office, said her office has since changed its policies and no longer allows pleas in drug cases until a lab report is completed. There are probably about 200 of these cases still pending in the DA's office, she said.

"That's what I'm tasked with as a prosecutor, to seek justice, not to seek convictions. Justice is making sure the wrong person doesn't go to prison just as much as it's making sure the right person does," Chandler said.