Updated

The startling scenes this week of heavily armed police clashing with protesters in a St. Louis suburb have galvanized a bipartisan push in Washington to challenge a controversial Pentagon program that gives away free military gear to local law enforcement.

Concerns about the so-called “militarization” of local police have been simmering for some time. But the clashes in Ferguson, Mo., over the death of an unarmed black teenager illustrated precisely what lawmakers have been worried about – police using a “military response” to subdue civilian riots and protests, and potentially escalating the situation in the process.

Sen. Rand Paul, R-Ky., warned in an op-ed that the scenes from Ferguson – which has since settled somewhat after state Highway Patrol took over security – resembled “war more than traditional police action.”

The concerns over heavily armed police have brought together a complex cross-section of libertarian Republicans and liberal Democrats in the same way as recent controversies over drones and National Security Agency snooping.

On the Democratic side, Rep. Hank Johnson, D-Ga., said he plans to introduce legislation when Congress returns in September to curb this type of gear build-up for local police forces.

"Our Main Streets should be a place for business, families and relaxation, not tanks and M16s," Johnson said Thursday. "Militarizing America's Main Streets won't make us any safer, just more fearful and more reticent."

Ferguson Police Chief Thomas Jackson, though, defended his department's response to the at-times violent protests in an interview with Fox News on Thursday.

"During this whole thing, these several nights of riots and all the arrests that we've made and all the conflict, not a single protester has been injured," Jackson said, claiming shots had been fired at officers during the protests.

But critics say the military-style presence raises tensions. Local police have been snatching up this equipment from the Defense Department under a 1990s-era program that transfers excess property to state and local law enforcement without charge. It’s a way for the military to put unneeded equipment to use without having to destroy it in the field.

Not all the equipment is war-style gear. The program transfers everything from copiers to fax machines to filing cabinets.

But it also transfers M-16s and heavy-duty tactical vehicles – two of those, both Humvees, ended up in the hands of the Ferguson Police Department.

According to the Defense Department, a total of 8,000 agencies at all levels of government participate in the so-called 1033 program. Since its start in 1997, the program has transferred more than $4.3 billion in equipment to these agencies. Last year, nearly $450 million worth was transferred.

Asked about the program on Thursday, Pentagon spokesman Read Adm. John Kirby called it a “useful program that allows for the re-use of military equipment that otherwise would be disposed of that can be used, again, by law enforcement agencies to serve their citizens.”

“The program serves a purpose,” he said, adding that “it is up to law enforcement agencies to speak to how and what they gain through this system.”

Attorney General Eric Holder said he's concerned that use of military equipment by police in Ferguson is sending a "conflicting message." Holder said authorities there have accepted the Justice Department's offer of crowd-control help as it continues to investigate the Saturday shooting death of 18-year-old Michael Brown.

Paul blamed the trend on the federal government, in his column in Time.

"Washington has incentivized the militarization of local police precincts by using federal dollars to help municipal governments build what are essentially small armies -- where police departments compete to acquire military gear that goes far beyond what most of Americans think of as law enforcement," Paul wrote.

The Associated Press contributed to this report.