(CNN) First, a Cleveland police officer shot and killed 12-year-old Tamir Rice as he played with a toy pellet gun in a public park.

Then Cleveland asked the Rice family to pay for his final ambulance ride.

That development, coming on Wednesday nearly 15 months after Tamir's death, stirred up fresh outrage -- not just from his family, but from the Cleveland police union that has fought them tooth and nail to clear the officers involved in the shooting.

"Subodh Chandra and I have never agreed on anything until now," police union President Steve Loomis told CNN affiliate WJW , referring to the Rice family's lawyer. "It is unconscionable that the city of Cleveland would send that bill to the Rice family.

"Truly disappointing, but not at all surprising."

As furor over the city's move grew Thursday, officials said they were withdrawing the $500 claim that was filed against Tamir's estate for the ambulance ride. Mayor Frank Jackson insisted there was "no intent" to make Rice's family pay for the ambulance trip, explaining that a city bureaucrat simply had followed routine.

'Adds insult to homicide'

It's an explanation the Rice family's attorneys say they don't believe.

"This is the biggest case in the city of Cleveland. You know, it's not often that a 12-year-old boy ends up getting shot by the police, and for them to say this was just part and parcel of, you know, what they typically do, I just don't buy it," attorney Earl Ward said. "This is not a routine case, and to say it was handled in a routine way just to me doesn't cut it."

Chandra also had harsh words for city officials.

"The callousness, insensitivity and poor judgment required for the city to send a bill ... is breathtaking," Chandra said. "This adds insult to homicide."

Samaria Rice, he said, feels the $500 claim and invoice for her son's ambulance ride amount to harassment.

Tamir Rice was 12 when he was shot and killed by police.

Believing boy had real gun, police shot him in seconds

It all goes back to November 22, 2014, when Tamir was in a park yards from his Cleveland home. Surveillance video showed him walking back and forth -- alone, it seemed -- occasionally pointing his pellet gun.

One man noticed him and phoned 911. "The guy keeps pulling it in and out," the caller said of the toy gun. "It's probably fake. But you know what, he's scaring the s**t out of people."

The part about the gun probably not being real never filtered down to police.

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Instead, a police car rapidly pulled up on snow-covered grass near a gazebo in the park, two officers got out, and one of them -- fearing, according to his defenders, the boy would fire -- shot Tamir within two seconds

He died the next day.

Mayor: City did its duty by sending out invoice

A lot has happened since, including large-scale protests and numerous legal proceedings.

In December, a grand jury declined to indict the two responding police officers in the case. A federal review of the case is ongoing, and the Rice family is still pressing its case, including with a wrongful death lawsuit against the city of Cleveland.

The city, in its 41-page response to that suit, claimed Tamir died because he failed "to exercise due care to avoid injury."

The ambulance invoice came about as part of legal proceedings, according to Cleveland's mayor. He said the city had written off the $300-plus that Medicaid did not cover from the ride and closed the account.

The estate's executor, Cleveland chief corporate counsel Rick Horvath explained, was doing his or her "legal duty" by reaching out to the city as a "potential creditor," seeking a statement "on any outstanding charges."

A Cleveland official then reopened the account and sent out the invoice.

"They have a duty and responsibility by law to ask for it," Jackson said, citing a policy standard for the $12 million to $15 million cost for ambulance services his city deals with each year. "And, when they ask for it, we have a duty and responsibility ... to give it to them."

Flags should have been raised given the high-profile incident, Jackson said.

"It was a mistake in terms of us not flagging it," Jackson told reporters. "But it was not a mistake in terms of the legal process. It followed the legal process."

The mayor said the city never planned on recouping anything from the Rice family, stating, "That account is going to be closed again and we're withdrawing that claim." He apologized if the episode "added to any pain or grief" Tamir's relatives may have.

Jackson also said his administration will investigate what happened, trying to determine if "there was any gross negligence involved or some intent" by a city official.

At the same time, he insisted the situation wasn't the city's doing in the first place.

"The only reason we sent the bill to the estate is because they asked for it," the mayor said. "If they had not asked for it, then we would not be having this conversation."