The chain of events that ended Tuesday night with the fatal shooting of a New York police officer began less than 15 minutes earlier with a radio call for help, officials said Wednesday.

It started at 8:30 p.m. with a pair of housing police officers on a rooftop who witnessed a shooting on the street and called for assistance, said William Aubrey, NYPD's chief of detectives for Manhattan.

Some 18 blocks away, Officer Randolph Holder and his partner heard the call.

The communications on the radio informed officers that one of the suspects -- later identified as Tyrone Howard -- had fled north, and to aid his escape stole a bike at gunpoint from a bystander.

Holder and his partner were 16 blocks away from where the bike was stolen.

According to Aubrey, Holder heard the transmission about the shooting and stolen bike, and he and his partner followed three people who investigators later determined had nothing to do with the shooting or the theft.

At that point, Howard -- the suspect on the bike -- came across the officers, put the bike down and pulled out a gun, Aubrey said.

He allegedly shot Holder in the head. Holder died hours later.

The 30-year-old suspect then moved toward Holder's partner, who drew his gun and fired, police said.

Other officers apprehended Howard four blocks uptown.

He was charged Wednesday with first-degree murder and first-degree robbery, and was expected to be arraigned Wednesday night, police said.

Holder was the fourth officer killed in the line of duty in the city in the past 11 months, according to police Commissioner William Bratton. Nationwide, 30 police officers have been killed in the line of duty so far this year.

Why was suspect on the streets?

Howard was wanted in connection with a gang-related shooting on September 1 and had a warrant out for his arrest, according to NYPD Chief of Department James P. O'Neill.

The Manhattan Violent Felonies Squad has been on the case and made numerous attempts to arrest Howard, O'Neill said. The squad went to a court appearance that Howard was scheduled to attend, but he didn't show up in court, he said.

"We were hunting him," O'Neill said, "but obviously unsuccessful."

Howard has been in and out of jail, Bratton said, and was arrested last year in a "major operation" in which 19 individuals were arrested.

He was twice recommended for a diversion program in 2014, instead of jail time, but never attended the programs, a law enforcement source told CNN.

Speaking to reporters after the shooting, Bratton criticized court officials for recommending Howard for the programs, which give offenders who face nonviolent charges the chance to attend treatment programs instead of jail or probation.

"If there was ever a candidate not to be diverted, it would be this guy," Bratton said. "He's a poster boy for not being diverted."

New York Mayor Bill de Blasio also said the suspect should have been behind bars.

"When you look at the offenses that this individual committed, they were consistent, they were violent," de Blasio said. "I don't know the details of how his case was handled by the judge, but I do know that someone like this shouldn't have been on the streets."

Howard was facing charges for nonviolent drug-related offenses when a New York judge recommended the diversion program, the law enforcement source said.

The judge gave him a second chance when he did not appear at the program the first time, the source said. When he didn't show up again, the court issued a warrant for his arrest.

New York court spokesman David Bookstaver said the diversion programs exist for cases involving nonviolent drug offenders and those with addictions. If Howard hadn't been offered diversion, he also could have been out on bail awaiting trial.

Son, grandson of police officers

Holder was a native of Guyana, the son and grandson of police officers.

He followed his father to America and followed in his footsteps, becoming an officer in 2010.

"He had always wanted to be a policeman," Randolph Holder Sr. told CNN affiliate NY1 , describing his son as a proud member of the force.

The officer was about to close on a home with his longtime girlfriend, his father said, "but all the dreams went down the drain."

Malika Clarke-Yarde, a friend of the fallen officer, made a tearful plea to officials as she stood beside family members.

"Mr. Mayor, do something. Mr. Commissioner, do something. Do something!" she shouted. "Too many fatherless children. He was just a father standing up for truth and right."

De Blasio, speaking shortly after the shooting, said the city is in mourning.

"We are fighting to understand the loss of a man who did so much, cared so much, dedicated so much to protecting others," he said.

"An immigrant who wanted to give back to his city and his country. ... In the words of Abraham Lincoln, a man who gave the last full measure of his devotion on behalf of a city he loved."

Fallen officers in New York

Holder is one of four officers killed in New York in the past year.

Brian Moore was fatally wounded as he sat in an unmarked police car in Queens with his partner on May 2. The officers saw a man adjusting something in his waistband, and when they pulled up behind him to ask questions, the man opened fire on them, hitting Moore. His partner was uninjured.

On December 20, a gunman approached officers Rafael Ramos and Wenjian Liu as they sat in their car in Brooklyn and shot them both in the head. The suspect then killed himself.

Officer shot in Kentucky

In an unrelated incident, a police officer was shot during a standoff at a home Tuesday night near Prestonsburg, Kentucky. Officer Adam Dixon suffered gunshot wounds to the chest and was hospitalized. He was in stable condition, police said.

After the suspect, Robert Powers, allegedly shot Dixon, other officers returned fire. Powers barricaded himself inside a home, where officers then arrested him.

Powers was being treated for non-life-threatening gunshot wounds, police said, and has been charged with attempted murder of a police officer.