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A second suspect in an armed robbery that led to the shooting of a Milwaukee police officer and the fatal shooting of another robbery suspect has been arrested, Milwaukee police said Tuesday.

Also Tuesday, a search warrant revealed that investigators suspect the gun used to shoot the officer was obtained through a straw buyer at a Gander Mountain store in Germantown.

Burt T. Johnson, 38, was fatally shot about 8:30 p.m., Thursday when he opened fire on officers who began chasing him after seeing him and another masked gunman exit the O'Reilly Auto Parts store near N. 76th St. and W. Mill Road, according to Milwaukee police, who confirmed the arrest of a 20-year-old man in connection with the robbery.

A 38-year-old officer was shot during the exchange of gunfire, however the bullet struck his body armor and he suffered only "significant contusions," according to Milwaukee Police Chief Edward Flynn.

According to the search warrant, a black, .45 caliber Springfield Armory semiautomatic handgun was found next to Johnson's body, and its purchase was later traced to the Gander Mountain store.

The gun, along with a Smith & Wesson .40 caliber handgun, was purchased April 8 by a 30-year-old Milwaukee woman suspected of being a straw buyer — a person with a clean record who buys a firearm for someone prohibited from owning one because of a criminal record.

Johnson had previous convictions for burglary and armed robbery, according to the warrant, which police obtained in the hope it could provide evidence of a straw purchase.

The case against Johnson's alleged accomplice will be presented to the Milwaukee County District Attorney's Office in the coming days, according to police, who were not releasing details of the second suspect's arrest or their investigation Tuesday.