Politico said its review of documents -- obtained through the Freedom of Information Act -- indicated serious death threats against lawmakers dropped in the past 10 years, along a pattern similar to Congress's overall public approval.
"It's interesting that specific events and legislation can trigger death threats," said Darrell West, vice president and director of governance studies at the Brookings Institution. "The popular image is that these people are crazy, not that they have policy motivations behind their anger. It's interesting to see that connection."
The documents indicated death threats investigated by the FBI hit a yearly high of 42 cases in 2001, the year as the terrorist attacks on the United States and when 56 percent of people approved of the job Congress was doing.
Any hot topic -- funding the war in Iraq, immigration, the outlawing of music download Web sites -- could trigger a threat, the review indicated.
By 2006, when the congressional approval rating dropped to an average of 25.4 percent, the FBI proved nine death threats.
"It seems what work was being done during that period of time deemed acceptable by the public may have been anathema to 'haters,'" Senate Sergeant at Arms Terrance Gainer said.
The FBI did not offer an opinion about why the number of threats dropped, Politico said.
"While the FBI cannot speculate on a specific reason for the decline in the number of threats directed against members of Congress since 2001, all threats and incidents are taken seriously and are investigated by the FBI, U.S. Capitol Police and law enforcement agencies around the country to identify and bring to justice those responsible," FBI spokeswoman Lindsay Godwin said.