WASHINGTON — Even before Saturday's tragic shooting in Arizona that left Rep. Gabrielle Giffords, D-Ariz., gravely wounded and a federal judge and at least five others dead, there were signs that threats to public officials were on the rise.

The threats and Saturday's horrific violence come at a time when many Americans, in the midst of heated debates over important issues, also demand that their members of Congress be available to them. Giffords was doing that in a "Congress on the corner" gathering Saturday at a Tucson grocery store, when a gunman opened fire.

A somber President Barack Obama, speaking from the White House, noted that Giffords was "listening to the hopes and concerns of her neighbors. That is the essence of what our democracy is all about. That is why this is more than a tragedy"

The shooting comes at the end of a week in which letters containing incendiary devices were sent to Homeland Security Director Janet Napolitano and Maryland elected officials.

Giffords' colleagues were urging caution, but also defiance.

"I hope it helps us be more mindful of security, but I hope it doesn't change a thing that we do in terms of interacting with constituents," Rep. Trent Franks, R-Ariz., told MSNBC.

He said "most members" of Congress had been threatened, and disclosed that some of his aides in Arizona have permits to carry guns for protection. Franks said the shooting was the work of a "monstrous degenerate," and that such threats will never be eliminated in a free society. A 22-year-old Arizona man was taken into custody in the case.

The U.S. Marshals Service in 2008 reported that threats to federal judges and prosecutors were up 69 percent from 2003, and that such threats were averaging nearly 100 a month nationwide.

Threats to other public officials, including members of Congress, are more closely guarded. But anecdotal accounts abound.

Last month, a San Francisco man, who was opposed to health-care reforms passed by Congress, was sentenced to 21 months in federal prison for threatening to destroy the home of then-House Speaker Nancy Pelosi. Appearing at her own public event in San Francisco Saturday, Pelosi expressed "deepest sadness for the act of violence."

On Dec. 30, a former firefighter pled no contest for threatening Sen. Barbara Boxer, D-Calif., Rep. Mary Bono, R-Calif., and other public officials. Prosecutors said the man was trying to set up his estranged wife by making it look like she was doing making the threat.

During the debate over health care, then Rep. Bart Stupak, D-Mich., an abortion opponent, was in the middle of tough negotiations to ensure that federal funds not go to abortions under the new law. Then negotiations put him at odds with people on both sides of the issue.

Stupak said he "received so many death threats that I was advised to get a security escort around Washington. My wife, Laurie, has had to unplug our home phone to avoid drunken messages from people screaming, swearing and generally acting profane - usually around the time the bars in their states close."

Writing in Newsweek, Stupak added: "One day I got 1,500 faxes, all hate mail."

The former police officer decided not to seek re-election.

Former Democratic National Chairman Howard Dean said that he wore a bulletproof vest when he was governor of Vermont and his state was moving toward legislation legalizing civil unions for gay couples in 2000.

Perhaps the most serious recent attack on members of Congress before Saturday came in late 2001, when the Senate majority leader of South Dakota and Sen. Patrick Leahy, D-Vt., along with some media figures, received letters containing deadly anthrax spores.

A government scientist suspected of sending the anthrax committed suicide in 2008.

Like Saturday's killings in Arizona, some bystanders paid the ultimate price in those attacks. Five people who handled the letters died, and 17 others were infected.