An exasperated President Barack Obama on Tuesday expressed new surprise and dismay at the lack of congressional action on stemming gun violence, hours after yet another American community was jolted by a deadly school shooting.

He said the failure to expand background checks on firearms sales was his "biggest frustration" as president, and called on concerned citizens to vent their anger loudly.

Obama said it was "stunning" Congress couldn't pass laws making it harder to obtain firearms in the aftermath of the 2012 elementary school shooting that devastated Newtown, Connecticut, and stunned the nation.

Obama said the violence on that December morning amounted to the "worst day" of his presidency.

"Why aren't we doing something about this?" he asked during an online question-and-answer session on college costs at the White House.

"I will tell you, I have been in Washington for a while now. Most things don't surprise me. The fact that 20 six year olds were gunned down in the most violent fashion possible and this town couldn't do anything about it was stunning to me," he said.

Obama's remarks came hours after a student opened fire at a high school near Portland, Oregon, killing a classmate before turning the gun on himself.

The incident joined a list of dozens of shooting incidents at U.S. schools, campuses and public areas that have taken place since the 2012 Newtown massacre. Just Sunday, two police officers in Nevada were shot and killed while eating lunch.

"The country has to do some soul searching about this," he said. "This is becoming norm."

Ramping up restrictions on gun sales was a top priority of Obama's after Newtown.

A bipartisan measure that would have mandated background checks on gun sales failed to pass the Senate the following April, and other proposed measures, like a ban on assault weapons, garnered little support among lawmakers.

The failure of it all was seen as a major defeat for the President, who led the nation in mourning for the students and teachers killed in Connecticut.

Without congressional support, Obama signed dozens of unilateral executive actions meant to quell gun violence. Though broad action, like banning assault weapons or high capacity magazines, still require congressional approval.

On Monday, Obama said the United States was the only developed country on Earth where mass shootings occur with regularity, adding that "if public opinion does not change, we're not going to change."

And he shot down arguments that mental health was more to blame for mass killings than access to guns.

"The United States does not have a monopoly on crazy people. It's not the only country that has psychosis," he said. "And yet we kill each other in these mass shootings at rates that are exponentially higher than any place else. Well, what's the difference? The difference is that these guys can stack up a bunch of ammunition in their houses and that's sort of par for the course."

Politicians from both parties are "terrified" of the National Rifle Association, Obama said, saying the gun rights organization was well-funded and able to influence elections to prevent any significant new gun regulations from becoming law.

Late in the day on Capitol Hill, Democratic Rep. Earl Blumenauer stood before the chamber and could barely get the words out about the violence earlier in the day in his district at Reynolds High School in Troutdale.

"In a scene that is achingly familiar, this morning at Reynolds a shooting occurred, a student killed, the shooter died, a teacher wounded," he said.

The chamber then observed a moment of silence.

(CNN)