He was just 29 years old.

Johnson was playing in an Elite Ice Hockey League game for the Nottingham Panthers in England when the horrifying moment happened.

As the ex-Pittsburgh Penguins center was taking the puck across the blue line, an opposing player on the Sheffield Steelers kicked his skate into the air and struck Johnson in the throat area. Johnson was rushed off the ice and received immediate treatment ... but he later succumbed to his injuries.

South Yorkshire Police said officers were on the scene and are currently investigating "the circumstances surrounding the incident."

"We would encourage the public to avoid speculation regarding the incident while we continue our enquiries," the SYP said.

The Panthers, meanwhile, called the moment "a freak accident" ... adding that they're "truly devastated" over it all.

"The Panthers would like to send our thoughts and condolences to Adam's family, his partner and all his friends at this extremely difficult time," the team said. "Everyone at the club including players, staff, management and ownership are heartbroken at the news of Adam's passing."

Johnson played for the Penguins in the 2018 and 2019 seasons -- logging a goal and three assists in 13 games. He went on to play professionally overseas ... and had just signed on to skate for the Panthers this season.

"The Pittsburgh Penguins join the entire hockey world in mourning the loss of Adam Johnson, whose life tragically ended far too soon," the Penguins said in a statement Sunday. "We offer our deepest condolences to Adam's family and friends, as well as all of Adam's past and present teammates and coaches. Adam will always be a part of the Penguins family. It was our honor to watch him fulfill his dream of playing in the National Hockey League."

The Panthers announced Sunday their game against the Glasgow Clan on Tuesday has been postponed in the wake of Johnson's death. Further changes to the schedule were being considered, the team added.