A man has been detained after accidentally fatally shooting his co-worker by pushing a button on what he thought was a pen but turned out to be a firearm.

The pair found the “pen gun” on Thursday night stashed inside a wallet that had been left in rubbish bin in Phnom Penh’s Sen Sok district.

“[The man] saw a small button on it and pressed it,” said Mak Hong, Sen Sok district police chief. “It shot his co-worker below the armpit. The victim was taken to hospital but died on the way.”

Hong identified the victim as Thy Theang, 28, though refused to identify the suspect.

He said police were investigating a soldier’s identity card also found in the wallet.

Colonel Eng Sophea, chief of the Municipal Serious Criminal Police Unit, said the implement, the size of a normal pen, was imported and used in “spying activities”.

“He did not know it was a pen gun. He pushed its button and it blasted, shooting his friend,” Sophea said.