Investigators in Arizona have said their efforts to clear a backlog of more than 6,400 rape kits have led to a slew of arrests and convictions.

Prosecutors in Maricopa County and police in Tucson and Tempe said testing on more than 5,000 backlogged rape kits led to more than 30 arrests and 21 convictions, the Arizona Republic reported last week.

A rape kit collects evidence that can lead to a suspect through DNA.

The testing has been conducted with grants topping $3.2 million in all, from the Manhattan District Attorney’s Office in New York. Maricopa prosecutors said they got another $2.7 million grant from the U.S. Justice Department to finish the job of testing backlogged rape kits and to hire staff members focused on sex-assault cases.

TESTING OF 100K BACKLOGGED RAPE KITS ACROSS US LEADS TO 1,000 ARRESTS

The rape kit backlog has been nearly cleared in Maricopa and cleared completely in Tempe.

Tucson police are now sorting through more than 400 hits, according to the paper.

"What we found immediately after testing kits from the (district attorney of New York) grant was that DNA pops up in multiple results and this person who pops up in multiple kits is a serial rapist," Detective Dallas Wilson said. "That -- coupled with a better understanding of the effects and memory -- has really changed the way we do sexual assault investigations."

The Arizona Sexual Assault Evidence Collection Kit Task Force reported a backlog of 6,400 untested kits statewide in 2016.

VIRGINIA AG: TESTING ELIMINATES PRE-2014 RAPE KIT BACKLOG

Some of the cases dated back decades, the paper reported.

Testing in 2017 on one backlogged rape kit led Maricopa prosecutors to Nicholas Blackwater, a man serving a 54-year prison sentence for a series of sexual assaults from 1997 to 2001, Cronkite News, the news division of Arizona PBS, reported last year.

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The testing on a 17-year-old rape kit tied Blackwater to a series of rapes dating back to 2000, the news outlet reported. He pleaded guilty to kidnapping with sexual motivation. His sentence was an additional four years in prison.

Tasha Menaker, of the Arizona Coalition to End Sexual and Domestic Violence, told Cronkite News that clearing the backlog “will bring justice to a lot of people whose cases were previously uninvestigated.”

The Associated Press contributed to this report.