Anna Gullberg who leads the editorial staff for Gefle Dagblad, a daily based in the mid-sized city about 200 km north of Stockholm, told Radio Sweden that someone associated with the mosque, though it's unclear who, made the threat during a telephone call with the chief of police in Gävle to complain about the paper's reporting.
Police have not shared details about the threat with the media but have confirmed that a preliminary investigation was launched.
"It was a close relative of the imam who was very upset with our reporting as I understand it," said Gullberg. She said she hadn't heard the details of the threat herself, but it was enough to prompt the police chief to file charges.
Gefle Dagblad began publishing stories about leading members of the mosque who they reported preach an extremely conservative interpretation of Islam.
In February, two days before the phone call and death threat, the newspaper had published an article about a board member of the mosque who had uploaded pictures of the Islamic State's flag on his Instagram account. The newspaper reported that in connection to the image of the flag there was written, "We Muslims love, honor, and raise this flag."
In a message on the mosque's website, the board member wrote that the flag was not a symbol created by the militant group but that rather it was a religious symbol co-opted by IS.
Leaders at the mosque have also reported the paper for defamation to police last year over articles written about their alleged connections with a website that advocated extreme viewpoints, like that homosexuality should be punishable by death.
Gullberg said that members of the mosque have criticized the newspaper's coverage for being racist, deceitful, and unfair. And she said that she believes the threat was made in a conversation meant as a complaint about the newspaper's story.
"I don't feel scared. I couldn't have my job if I walked around feeling scared," she said. "I'm really angry that this thing can happen."