President Obama opened the door Monday to re-evaluating a massive Defense Department program that sends military equipment to local police forces, in the wake of concerns about the St. Louis County Police Department's response to riots and protests in Ferguson, Missouri.

"There is a big difference between our military and our local law enforcement, and we don't want those lines blurred," Obama said.

The president spoke at the White House briefing room late Monday, during a pause in the capital from his vacation in Martha's Vineyard. As he has before, Obama walked a fine line in addressing the sustained violence and unrest in the St. Louis suburb over the killing of an unarmed black man by a police officer.

He said most demonstrators are protesting peacefully, but said a "small minority" are not. To those looting and attacking officers, Obama said their behavior "undermines, rather than [advances] justice."

Obama also said "there's no excuse for excessive force by police."

Noting that the Justice Department has opened a federal civil rights probe into the shooting death, he said Attorney General Eric Holder will travel to Ferguson on Wednesday to meet with FBI and DOJ officials on the ground, and local leaders.

But he also touched on what has become a controversial topic, in Washington and local communities across the country -- the alleged "militarization" of local police forces.

The concerns are connected to a U.S. military program that gives away free military gear to local law enforcement (including those in the St. Louis suburb). 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.

Some lawmakers, on both sides of the aisle, have questioned the program in the wake of the Ferguson unrest.

Obama said the program has provided some useful supplies to local agencies, including radios and equipment to respond to chemical attacks.

But he added: "I think it's probably useful for us to review how the funding has gone, how local law enforcement has used grant dollars to make sure that what they're purchasing is stuff that they actually need."

Obama said he thinks there will be "bipartisan interest in re-examining some of those programs."

Obama, meanwhile, expressed sympathy for the "passions and anger" sparked by the death of 18-year-old Michael Brown in Ferguson, but said giving in to that anger through looting and attacks on police only stirs tensions and leads to further chaos. He said overcoming the mistrust endemic between many communities and their local police would require Americans to "listen and not just shout."

"That's how we're going to move forward together, by trying to unite each other and understand each other and not simply divide ourselves from one another," Obama told reporters at the White House. "We're going to have to hold tight to those values."

Obama had a round of meetings in Washington on Monday, amid some criticism over his vacation schedule. He was briefed by Holder on the events in Missouri.

The meetings came as conditions in Ferguson deteriorated. Missouri Gov. Jay Nixon lifted a curfew but ordered the National Guard to step in to help restore order. Holder over the weekend ordered a federal medical examiner to perform a third autopsy on the teenager.

Obama said he has told Gov. Nixon he wants to ensure the use of the National Guard must be limited in scope, and said he would be monitoring that operation in the coming days to see whether the guard's involvement was helping or hurting.

The Associated Press contributed to this report.