US President Barack Obama riffed on the increasing prevalence of mass shootings in America on Tuesday, lamenting the inability of Congress to pass new laws aimed at reducing gun violence in the wake of numerous tragedies.

“Our levels of gun violence are off the charts,” Obama said. “There is no advanced developed country on earth that would put up with this.”

Obama’s extended remarks on gun violence came in response to a question at an event moderated by Tumblr founder David Karp. One questioner was from the University of California, Santa Barbara, where a shooter had killed seven people in a shooting spree just more than two weeks ago. On Tuesday, a shooting at a Portland, Oregon-area high school left two people dead, including a shooter.

Obama said the country should be “ashamed,” and called the failure to pass new laws the “biggest frustration” of his presidency thus far. He reflected on the December 2012 shooting at Sandy Hook Elementary School in Newtown, Conn., that killed 26 people, which he has repeatedly called the “worst day” of his presidency.”

“The fact that 20 6-year-olds were gunned down and this town couldn’t do anything about it was stunning to me,” Obama said.

“We’re the only country in the world where this happens,” he added, “and it happens once a week.”

Obama cited favourably Australia’s enactment of gun-control laws after a single mass shooting. Australia’s laws, and the drop in gun violence that followed them, have often been cited as an argument by gun-control supporters in the U.S. as a model.

Although Obama called again for measures like an expansion of background checks, which failed to pass the Senate last year, he lamented the reality — the votes for it in Congress are not there. He blamed the lack of votes on members of both parties being “terrified” of the National Rifle Association. To change gun laws, he said, a “fundamental shift in public opinion” would need to occur — one in which Americans vote out their members of Congress because of opposition to gun-control laws.

“This country has to do a lot of soul searching,” Obama said.

Obama also rejected arguments that the root cause of the violence is mental health, saying the U.S. doesn’t “have a monopoly on crazy people.”

“It’s not the only country that has psychosis, and yet we kill each other in these mass shootings at rates that are exponentially higher than anyplace else,” Obama said. “What’s the difference? The difference is these guys can stack up a bunch of ammunition in their houses.”

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