Note: Brandon Garrett, a professor at the University of Virginia School of Law, is the author of the forthcoming book, “End of Its Rope: How Killing the Death Penalty Can Revive Criminal Justice.” Here are his thoughts on the developments of last week.

By Brandon L. Garrett

Marcellus Williams, convicted of murder in Missouri, had his execution stayed last week by Gov. Eric Greitens pending further investigation of DNA evidence in the case. (Missouri Department of Corrections)

The American death penalty has a big innocence problem, and it is not going away.  The events of last week show why.

On Wednesday, Missouri planned to execute Marcellus Williams. The problem was that he may be innocent. Governor Eric Greitens wisely put that execution on hold while a panel investigates further.  On Thursday, Florida did execute Mark Asay.  We may never fully know whether he actually deserved the death penalty.

In the Williams case, although the courts said that the execution could go forward, the courts disregarded new DNA tests that show Williams’ DNA was not on the weapon that killed Lisha Gayle at her home in 1998. The DNA of another unidentified man was on the weapon. The victim was stabbed 43 times, and it stands to reason that the male DNA on the weapon is that of the actual culprit.

The state of Missouri said that the other evidence in the case is still strong. Yet that evidence consisted of the testimony of informants, both drug addicts, who received financial incentives to testify against him. The footprint at the crime scene and the hair samples from the crime scene do not match Williams either.

To be sure, Williams had a number of items belonging to the victim and sold a laptop belonging to the victim’s husband. That is strong circumstantial evidence.  Then again, those items were found by one of the cooperating informants, Williams’ girlfriend at the time. The case was built around the informants. Both had hoped to get a $10,000 reward.

The jury that convicted Williams never heard about the DNA evidence, and it is hard to imagine that if he was tried today that he would get a death sentence, given the new doubts about guilt. That DNA evidence has never been presented in court.

Compare the Asay case.  He fully admitted that he shot one of the victims, but in a fight over money, and not the type of murder that would likely qualify as so egregious that it deserves the ultimate punishment.  The evidence that put his case in the category of a death penalty case was testimony that he uttered a racial epithet when killing the victim and had white supremacist motives.  However, he denies ever having such views, and that evidence came from the same type of unreliable source as in the Williams case: a jailhouse informant.

It may surprise many people that such unreliable evidence is still used even in the most serious death penalty cases.  Today, there is much more awareness about wrongful convictions, including those due to false informant testimony. Polls show that more people are concerned about wrongful convictions and executions. Twenty people have been exonerated from death row based on DNA testing. Most of those individuals had allegedly made confessions, which we now know to be false, to police or to jailhouse informants.

Yet, that awareness has not stopped states from trying to execute people whose convictions are based on such flimsy evidence. Indeed, the more death sentences in a state, the more death row exonerations, as I describe in my new book, “End of Its Rope[hup.harvard.edu].” Florida, where Asay was just executed, leads the country in exonerations[deathpenaltyinfo.org] in death penalty cases.

Today, death sentences and executions are fading fast and one might think that we could limit the death penalty to the cases where we are sure that the person actually did it, with “it” being a murder serious enough to warrant the death penalty. Only 20 people were executed in 2016 and only 31 people were sentenced to death. Yet serious claims of innocence and unreliable evidence persist.

The evidence in death penalty cases is not always very strong. After all, in many murders, there are no surviving witnesses. Unfortunately, as a result, police sometimes cut corners to try to solve high-profile homicides, by relying on unreliable jailhouse informants or by coercing confessions from mentally ill individuals.

While we may desire speedy justice, new evidence of innocence may not surface until a decade or more after trial. One reason is that at the time of trial, the defense often has inadequate resources to investigate innocence or possible defenses. In Williams’ case, the defense lawyer admitted he was nowhere close to ready for trial and asked for more time to prepare, but the judge denied the request. I have found that the states with the most death sentences are the ones without law offices to handle the defense in death penalty cases.

The problem of innocence is inevitable and constant in death penalty cases. In April, Gov. Terry McAuliffe granted clemency to one of the few people left on Virginia’s death row, due to persistent doubts about his guilt. In contrast, Ledell Lee was executed in Arkansas earlier this year, despite strong claims of innocence.

And last week, the California Supreme Court decided to reject an interpretation of a new law that might have limited appeals to just five years, in favor of giving judges adequate time to carefully review death penalty cases.  After all, it takes much more time than that to properly investigate claims of innocence.

This will not be the first time that we have executed a man despite real doubts about the case. So long as we have the death penalty, it will not be the last.