Police and the FBI are investigating attacks and threats against Democratic members of Congress who voted for healthcare reform.

Democrats have complained over heated Republican rhetoric, including 2008 vice presidential nominee Sarah Palin's Twitter comment urging supporters, "Don't Retreat, Instead -- RELOAD."

A Palin Facebook post continued the firearms theme, featuring a map of the US targeting 20 members of Congress who backed the healthcare legislation, using the crosshairs of a gunsight to note each of their home states.

The House gave the healthcare measure, Barack Obama's top domestic priority, final congressional approval on Sunday. The President signed it into law on Tuesday, but emotions ran high among opponents and supporters during the yearlong debate over the legislation.

An FBI spokeswoman said the agency, the US Capitol Police and other law enforcement officers were investigating threats and incidents of violence against members of Congress.

House Majority Leader Steny Hoyer said they were concerned about "very serious incidents" that have taken place since Sunday's vote.

Some politicians have received death threats. Bricks were tossed through the windows of one House member's office in New York state, another was spat upon by a protester on Capitol Hill, while yet another was the target of a racial slur.

Michael Steele, chairman of the Republican National Committee, took aim at House Speaker Nancy Pelosi in an interview on Fox News, telling viewers, "let's start getting Nancy ready for the firing line in November."

The Republican Party's website features a picture of Pelosi in front of a wall of flames and urges visitors to the site to "make a donation to help us win 40 seats in 2010 and fire Nancy Pelosi."