Manhattan District Attorney Alvin Bragg‘s office failed to request a warrant to confiscate a gun used in an armed robbery by a serial offender — who then fatally shot a man with that same weapon exactly a month later, a union boss alleged.

Tyrell Rodgers, 29, allegedly pointed the silver revolver at his estranged girlfriend and stole her cell phone while threatening to kill her on Feb. 8, court records show. He thenallegedly killed a man with that revolver on March 8 — after Bragg’s office failed to issue a warrant that detectives on the robbery case requested.

“Why didn’t DA Bragg’s office act?” Detectives’ Endowment Association President Paul DiGiacomo asked. “Why did another New Yorker have to die?”

Rodgers, who has 23 prior arrests on his record, was charged with murder in the second degree, attempted murder in the second degree and criminal possession of a weapon for the March 8 killing, court records show. 

In the alleged February robbery, he pointed the silver gun at his girlfriend and her baby and said “give me the phone or I will kill you and everyone you love,” court documents show. Rodgers then took the cellphone and left the location.  

Detectives requested a warrant to look for the handgun after the robbery was reported on Feb. 17.DiGiacomo said the warrant never materialized.

Two and a half weeks later, Rodgers brought the gun to West 19th Street and Ninth Avenue around 10 p.m. to meet a man he had been arguing with over the phone, court records show.  

It wasn’t clear what the argument was about, but Rodgers fired the weapon and a bullet struck the man he had argued with in the right arm, records show. The man’s friend standing at his side was shot in the gut and pronounced dead about an hour later at Bellevue Hospital. 

Rodgers was arrested two days later on March 10 — the day the warrant was finally executed by Bragg’s office.

Detectives searched Rodger’s apartment at 321 East 153rd Street and quickly confiscated the gun that video surveillance had showed him firing, records show.It matched the description of the gun the girlfriend told detective Rodgers used to threaten her.

“Inside the apartment, I recovered a dark hooded jacket, dark mask and dark colored backpack that appear to be consistent with the type of clothing the shooter wore on video,” the detective wrote in court documents. “Inside of the dark backpack inside of a fanny pack I recovered a silver revolver.”

Rodgers was sent to Rikers Island on $100,000 cash bail or $100,000 bond.

A question to Bragg’s office asking what happened with the initial warrant request wasn’t answered.

But at Rodgers’ arraignment, prosecutors noted that his “girlfriend indicated to police that defendant frequently carries this weapon around.”

Additional reporting by Tamar Lapin