The federal government is investigating dozens of death threats to IRS employees that have been posted online since the House passed the health care bill, FoxNews.com has learned.

The health care law has sparked protests on radical anti-tax and anti-government Web sites and within their private, password-protected e-mail lists and message boards. Some writers have labeled March 21 -- the day the House passed the bill – "Bloody Sunday," and they see it as a call to violent action against IRS workers.

In the days following the House vote, animosity toward the IRS intensified, and many heated online protests included specific discussions about the best way to go about killing tax agents.

Hundreds of comments were posted in response to an incendiary story on infowars.com, the radical far-right Web site owned by radio host Alex Jones. The story, entitled, "The Cost Of Defying Obamacare: $2,250 a Month And IRS Goons Pointing Guns At Your Family," focused on the “increasing militarization of the IRS” and its expansion of powers under the new health care law.

One commenter wrote: "If they actually try to do this, there is going to be a whole lot of thugs start vanishing. This is the last line in the sand. Those fools have just signed their death warrants!!!"

"theres gonna be alot of IRS agents needing healthcare if they try to terrorize us Americans," another comment read. Yet another wrote, "Come and take them…….they will have to hire so many IRS agents because…well when 10 a day get killed….you do the math."

A federal probe has been launched into the comments. "We are actively investigating all threats made against IRS employees," J. Russell George, treasury inspector general for tax administration, said in a statement to FoxNews.com Tuesday morning.

Threats to IRS agents have increased steadily in recent years; more than 1,200 threat and assault cases between 2001 and 2008 were investigated.

In February, a small plane pilot named Joseph Stack published an anti-tax manifesto before he crashed his aircraft into a building in Austin, Texas, that housed an IRS office. Stack, who died in the attack along with an IRS worker, became a hero to many in the anti-tax and anti-government extremist movements; the inspector general launched investigations into at least 70 online comments made in support of Stack’s attack.

Now the agency’s expanded role under the health care law has sparked a new wave of animosity toward its employees, which experts say has contributed to an unprecedentedly volatile atmosphere.

"The anger and resentment within the anti-tax movement towards the new health care is overwhelming," said insurance analyst J.J. MacNab, who's testified before Congress about the anti-tax movement. "Combine this legislation with the census arriving in everyone’s mailboxes and the fact that we're heading into tax season, and this could be a really problematic month ahead."

The IRS is now working to beef up security across the country. It will be spending over $100 million in 2010 alone on office security, IRS Deputy Commissioner for Services and Enforcement Steven Miller told Congress earlier this month.

There are 755 IRS facilities in the U.S. — and 64 percent of the facilities have no security presence, according to recent congressional testimony by Colleen Kelley, the president of the National Treasury Employees Union. Union member have expressed concerns about the lack of security, she said, citing walk-in centers with no security other than a lock on the door.

One day after the House passed the health care bill, contract postings appeared on government employment Web sites requesting bids for round-the-clock armed guard security services for a half dozen facilities — including storage facilities and IRS grounds with child care centers. The IRS posted a request for quotes for 60 Remington Model 12 gauge pump-action shotguns for the Criminal Investigation Division, which was first reported on the Drudge Report last month.

According to an IRS office memorandum outlining policy for the use of armed guards during the current tax season, the inspector general will provide armed escorts under the following conditions in April:

- "When an IRS employee has contact with a taxpayer who is the subject of an open TIGTA-OI assault or threat investigation; or

"When an IRS employee has contact with a taxpayer who has been designated as a Potentially Dangerous Taxpayer (PDT) by the IRS Office of Employee Protection."

The IRS’s office of criminal investigations will provide armed escort assistance in other situations, the memo said.

But this has served only to further rile anti-tax extremists, who view themselves as patriots and pledge to confront any armed IRS agent with guns of their own.

"WE, I repeat, WE, are armed to the teeth. They need those pumps just to feel safe," one commenter wrote.

"POINT OF ORDER: I dont shoot BACK; I shoot FIRST……….BUY MORE AMMO," said another.

In a particularly heinous reply that was praised by dozens of other commenters, someone suggested – in graphic terms – that tax opponents should rape an attractive agent before killing her. "Or vice versa…don’t have a problem with that either."

Other online discussions delved into specific tactics—what weapons, what bullets, what bait—to employ in the killing of IRS agents.

Someone calling himself, "rEvolutionary" wrote, "I think it would take a different kind of bait. Humans are as predictable as any animal. What kind of bait would draw an IRS pirate..?"

"NOT LMAO"replied, "SITTING IN THE WOODS CRYING BECAUSE THEIR HOUSES ARE GONE SHOULD BE ENOUGH TO DRAW THEM IN….."

While these threats are being investigated, experts doubt there is much that law enforcement can do to predict if any of these commenters actually plan on taking action. The only thing certain is that the online community of anti-government extremists is growing, and it is increasingly being viewed by law enforcement as a threat.

"I've never seen anything like this before," said MacNab. “The health care bill could very well be the tipping point."