BRONX, N.Y. - The New York City Police Department is still trying to piece together the circumstances of Bronx officer Brian Mulkeen's death over the weekend.

Investigators say he may have been hit by friendly fire.

Mulkeen was on patrol at the Edenwald Houses when he and other officers stopped a man for questioning around 12:30 a.m. Sunday.

Sources have identified that person as Antonio Lavance Williams, 27.

Mulkeen struggled with Williams and shots were fired.

"Officer Mulkeen was on the ground wrestling with the suspect for an extended period of time, he’s yelling 'He’s reaching for it he’s reaching for it,' on the ground shots are fired," said NYPD Chief of Department Terence Monahan.

Mulkeen's weapon went off five times, but police are not sure who fired it.

Five other officers at the scene also fired, killing Williams.

A 32 caliber revolver was recovered at the scene.

Authorities say it belonged to Williams, but it had not been fired.

The 33-year-old had been on the job nearly seven years.

He is the second officer to be killed in the line of duty in 2019.

Mulkeen was also a 2008 graduate of Fordham University, where he was a track and field star and a championship weight thrower.

The school's president, highlighting Mulkeen's sacrifice, said, "Brian Mulkeen went out into the world to do exactly what we expect of our alumni—be a man for others—and he was slain in service to the local community."

Mayor Bill de Blasio has ordered all flags be flown at half staff until further notice, following Mulkeen's death.

He is asking all New Yorkers to keep his family in their thoughts.

"This is a young man who made the choice to join the NYPD, to put his life on the line, to protect others. When we met with his family, and one of the things they told us even with their grief, was that Brian made a choice, an incredibly noble choice," De Blasio said.

Meantime, the organzation Answer the Call, which provides funds to families of NYPD and FDNY members who have died, will donate $25,000 to Mulkeen's family to help with immediate expenses.