New York City Mayor Bill de Blasio called on Friday for changes to state laws that allowed a man charged with killing a police officer this week to remain on the streets even though he had a long criminal history that included drug arrests.

Tyrone Howard, who avoided prison by entering a court-mandated drug treatment program, was charged with murder in the shooting of New York City Police Department Officer Randolph Holder, 33, on Tuesday in the city's East Harlem neighborhood.

"The death of Officer Randolph Holder was a clear and tragic signal that we must ensure dangerous individuals with long criminal histories do not walk our streets," de Blasio said.

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Howard, 30, was arrested in a major drug operation last year for selling crack cocaine. He already had a string of drug-related arrests.

Two judges agreed to send his case to a drug court in Manhattan, one of thousands across the country that offer treatment, instead of incarceration, for defendants with no prior violent felony convictions.

Howard failed to complete the treatment and was being sought by police in connection with a Sept. 1 gang-related shooting at the time Holder was slain.

The killing has put the drug court system under greater scrutiny, even as advocates defend it.

De Blasio said judges should be required to consider public safety and flight risk when determining an individual's eligibility for a drug diversion program. Under current law, judges may consider all factors but are not obligated to take them into account.

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In addition, de Blasio said judges should be permitted to consider dangerousness when setting bail. New York is one of only three states that bars judges from doing so. Instead, they can only assess flight risk.

It is not clear if either of the changes de Blasio is proposing would pass the state legislature. 

New York State's chief judge, Jonathan Lippman, has repeatedly called for changes to the bail statute, without success. He announced his own reforms on Oct. 1, including appointing special judges to review every misdemeanor case in which a defendant has not made bail.

Last week, de Blasio announced creation of a "Bail Lab" to study ways to provide relief for defendants who remain in jail for months, or even years, because they cannot afford bail.

Taken in tandem with Friday's proposals, the reforms would aim to avoid unnecessary detention for low-level defendants while helping to keep criminals deemed to be dangerous behind bars, de Blasio said.