(CNN) Debra Milke spent 22 years on death row. She was convicted of conspiring to kill her son for an insurance payout.
The problem is, she didn't do it. In March, a judge ruled the Arizona woman innocent and dismissed all the charges against her.
Milke's case is one of a record 149 known exonerations in the United States and its territories in 2015, according to a Wednesday report from the National Registry of Exonerations at the University of Michigan Law School. On average, the exonerated defendants had served more than 14 years in prison.
Exonerations are now common, the report says.
"Not long ago, any exoneration we heard about was major news. Now it's a familiar story," it says. "We average nearly three exonerations a week, and most get little attention."
Homicides and drug cases made up more than two-thirds of the total.
The reasons for the exonerations are varied. Many involved official misconduct or cases in which defendants pleaded guilty, and some involved false confessions. But half were cases in which no crime occurred, according to the report.
Although a record number, the cases are a mere fraction of the millions of arrests made each year in the United States.
FBI crime statistics show an average of 12 million arrests per year from 2010 to 2014. The number of arrests is on a downward trend, as is violent crime over the five-year period.
The report attributes the growing number of exonerations to the growing use of conviction integrity units, which work to "prevent, identify and correct false convictions."
Still, the report calls the exonerated cases a "drop in the bucket." The registry has accounted for all known exonerations since 1989.
"By any reasonable accounting, there are tens of thousands of false convictions each year across the country, and many more that have accumulated over the decades," the report says.
"We've made a start, but that's all."