Two letters that contained threats to Mayor Michael R. Bloomberg — one addressed to him, the other to a lobbyist who works on his gun control campaign — have tested positive for the deadly poison ricin, the authorities said on Wednesday.

The first letter was opened at a New York City mail center in Lower Manhattan on Friday, the police said. Although staff members at the mail center do not appear to have become ill, several police officers who came into contact with the letter’s contents “indicated some mild symptoms the next day, including diarrhea,” and they are being treated in hospitals, the New York Police Department’s spokesman, Paul J. Browne, said on Wednesday afternoon. “They’re being checked out as a precaution.”

The second letter, which was opened on Sunday in Washington, was addressed to Mark Glaze, the director of Mayors Against Illegal Guns, a group Mr. Bloomberg helps run and finances, officials said. Mr. Glaze opened the letter, an official said. No injuries were reported, Mr. Browne said.

Both letters were identical in content, bore references to the debate over gun regulation and contained written threats to Mayor Bloomberg, Mr. Browne said. “It’s threatening, and it’s very clearly about one issue,” one official said of the letters.