The letter was intercepted by a White House mail screening facility. The Secret Service said the letter has been turned over to the FBI's Joint Terrorism Task Force for testing and investigation.
A law enforcement source told ABC News that the letter to the president had an anti-gun control message along with a suspicious substance, similar to two letters recently sent to Bloomberg. The Secret Service confirmed the apparent link among the letters.
The two letters addressed to the mayor had a Shreveport, La. postmark, according to a statement on the postal union's website.
One letter was sent to Bloomberg in Manhattan. It was intercepted at a mail screening facility near his office.
The second letter targeted Bloomberg's gun control group, Mayors Against Illegal Guns. That letter was opened on Sunday in Washington at the nonprofit group's office.
"The letter obviously referred to our anti-gun efforts. There's 12,000 people that are going to get killed this year with guns and 19,000 are going to commit suicide with guns, and we're not going to walk away from those efforts," Bloomberg said.
The letters did not harm any of the intended recipients, but three emergency responders who examined one of the letters did experience minor symptoms.
Both letters to the mayor contained threats referencing the debate on gun laws. The FBI is investigating.
The Associated Press and ABC News contributed to this report.
