CLICK IMAGE for slideshow: Riot police stand guard as demonstrators protest the shooting death of teenager Michael Brown in Ferguson, Missouri August 13, 2014. Police in Ferguson fired several rounds of tear gas to disperse protesters late on Wednesday, on the fourth night of demonstrations over the fatal shooting last weekend of an unarmed black teenager Brown, 18, by a police officer on Saturday after what police said was a struggle with a gun in a police car. A witness in the case told local media that Brown had raised his arms to police to show that he was unarmed before being killed. (REUTERS/Mario Anzuoni)

Police officers responding to protests in a St. Louis suburb Wednesday night were outfitted in fatigues, wore gas masks and body armor, carried military-style rifles, and were backed by tanklike armored vehicles as they sought to clear the streets.

Tear gas, smoke bomb explosions and the pop-pop-pop of nonlethal projectiles added to the picture, as photographs and video from Ferguson, Missouri, depicted a scene more reminiscent of a war zone than a civil rights protest against the police shooting Saturday of an unarmed teen in the largely low-income Midwestern town of about 20,000 people.

The military appearance of the St. Louis County police prompted an outpouring of responses from veterans and policymakers on social media and in statements. Brandon Friedman, a U.S. Army veteran, tweeted a photo of himself deployed in Iraq next to an image of a police officer in Ferguson. “The gentleman on the left has more personal body armor and weaponry than I did while invading Iraq,” Friedman wrote.

The gentleman on the left has more personal body armor and weaponry than I did while invading Iraq. pic.twitter.com/5u6TxyIbkk — Brandon Friedman (@BFriedmanDC) August 14, 2014

Democratic Rep. Hank Johnson of Georgia announced that he will introduce a “Stop Militarizing Law Enforcement Act” in Congress next month. And the issue even attracted a rare moment of bipartisan concern, as Republican Sen. Rand Paul wrote Thursday that local police departments are now “essentially small armies.” Attorney General Eric Holder, meanwhile, said in a statement that he was "deeply concerned" about the the "deployment of military equipment and vehicles" in the town.

So how did small-town cops end up with tanks and so much other military gear?

The vast majority of America’s police departments have special paramilitary units — called SWAT teams — to respond to emergency situations, conduct drug raids and even, as we’re seeing in Ferguson, patrol the streets and control crowds. In the past few years, more of these SWAT teams have received armored vehicles and other military-grade equipment provided for free by the federal government to expand their capabilities.

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The first SWAT team was formed to respond to the Watts race riots 50 years ago near Los Angeles, and the command structure soon spread to other police departments as the federal government began funding aggressive local responses to the “war on drugs” in the 1980s. After the Sept. 11, 2001, attacks, the federal government funneled even more money to municipalities for equipment to battle terror threats.

The Defense Department’s 1033 program, which began in the late 1980s to recycle old military equipment to local police, has given out tens of thousands of machine guns, military fatigues, and, more recently, at least 600 MRAPs (mine-resistant ambush protected vehicles) to help outfit SWAT teams. Police departments can also apply for grants through the Department of Homeland Security to buy lighter armored vehicles, like BearCats, and other military equipment to combat terrorism and drug dealing.

Police departments in St. Louis County have received equipment from the Pentagon program, including six Humvees, 12 M-16 rifles, and a bomb-defusing robot, according to DoD spokesman Mark Wright. The Bearcat, however, was not given to the county by the 1033 program.

A riot police officer aims his weapon while demonstrators protest the shooting death of teenager Michael Brown, in Ferguson, Missouri, on Aug. 13, 2014. Police in Ferguson fired several rounds of tear gas to disperse protesters late on Wednesday, on the fourth night of demonstrations over the fatal shooting last weekend of Brown, 18, an unarmed black teenager by a police officer on Saturday after what police said was a struggle with a gun in a police car. A witness in the case told local media that Brown had raised his arms to police to show that he was unarmed before being killed. (REUTERS/Mario Anzuoni)

Thanks to these programs, the presence of fully equipped SWAT teams in small-town America has become the norm. Eighty percent of small towns had SWAT teams by 2005, up from just 20 percent in 1980, according to research by criminology professor Pete Kraska of Eastern Kentucky University. More than 90 percent of city police departments have the special units.

The federal government argues that giving local police tanks and other leftover war equipment is a great way to avoid the waste of throwing away expensive gear that taxpayers have already paid for. But critics counter that militarizing police forces escalates conflicts and creates needless violence. Nearly 50 civilians were injured in 818 SWAT raids over two years in 11 states, the ACLU found in a June report. Only seven percent of the SWAT deployments the ACLU studied were responses to hostage or active-shooter scenarios — the majority were for drug raids.

It’s unclear how many armored vehicles have been used in Ferguson since the protests began Saturday. But at least one of the deployed vehicles, which appears to be a BearCat, had a large military-style rifle attached to the top, with an officer handling it. Missouri Gov. Jay Nixon recently said he was ordering the St. Louis County Police Department out of Ferguson.

The clashes began after police shot a black 18-year-old, Michael Brown, saying he had grabbed for a white officer’s weapon. They have declined to release the name of the officer who shot him, angering residents. A friend of Brown’s who was with him at the time of the shooting said both he and Brown were attempting to run from police, without struggle, when an officer shot Brown. President Obama announced Thursday that he is ordering the FBI and Justice Department to investigate the shooting.