Concerning details emerge about NYC gunman's football career as investigators probe if he was targeting NFL office

The killer gunman who opened fire inside the Manhattan building that houses the NFL was once a promising high-school football player, it has been revealed.

Authorities have identified Shane Tamura, a 27-year-old licensed private investigator from Las Vegas who once dreamed of a life in football, as the shooter who carried out a deadly rampage in the heart of New York City on Monday night.

Tamura walked into the lobby at 345 Park Avenue in Midtown Manhattan and sprayed a long-form M4 rifle, shooting an NYPD officer in the back, before heading to the offices of Rudin Management on the 33rd floor and continuing the terrifying spree, killing a total of four people. He then took his own life, police have confirmed.

The building at 345 Park Avenue is also home to the NFL headquarters, which is located on the fifth floor.

Sources confirmed Tamura did not enter that floor, yet investigators are looking into into whether he was targeting the NFL offices given his past as a high-school football standout for Granada Hills Charter and Golden Valley in California.

While he resided in Las Vegas, Tamura spent his high-school years in Los Angeles. He graduated from Granada Hills Charter, a Los Angeles Unified School District school located in the heart of the San Fernando Valley, in 2016. Prior to that he attended Golden Valley, which is further north in the Santa Clarita Valley.

According to his high-school football statistics websites MaxPreps and Hudl, Tamura operated as a defensive back, running back and halfback throughout his career.

In a video posted online from the 2015 season, he can be heard giving a post-game interview in which he spoke of his victory with the Granada Hills team.

Shane, who was instrumental in the win after scoring several touchdowns, said in the video: 'We were down 10-0, stayed disciplined and came together as a team. Couple of touchdowns.'

It is unclear whether his football career continued after he graduated from Granada Hills Charter in 2016.

DailyMail.com has approached the school's athletics department for comment.