Police are investigating a message on Bolton Wanderers and England Under-21 striker Marvin Sordell's Facebook page.
A mock-up photograph showed Bolton's Sordell with a gun pointed at his head. It showed blood across his face and chest and the message: "dont f*** with the millwall."
A Scotland Yard spokesman said: "No formal allegations have been made to us but officers have been investigating the claims made through the club media and social media. Investigations still continue."
The page appeared earlier this month after Millwall beat Bolton 2-1 and Sordell claimed on Twitter that he and three team-mates had been subjected to racist chants.
The creator, believed to be a 14-year-old teenage Millwall fan, has now sent Sordell a letter of apology, according to reports.
The teenager's father told The Bolton News: "He is really sorry about what he has done. He has written Marvin Sordell an apology.
"He was with his school friends and they got upset that Millwall had been given a bad press and he has just made it worse.
"It was all a big prank and he just thought it was a joke."
He added: "We told him what he had done had made the whole situation a lot worse.
"He is remorseful. He didn't realise the consequences of it but he is realising them now.
"He is normally as quiet as a mouse. He isn't a hooligan. He only goes to Millwall when I take him.
"We are shocked he has done this."
