"The NRA is full of shit," Rivera tweeted shortly after the shooting.
Rivera, a frequent critic of President Barack Obama, praised the president for his repeated calls for gun reform.
"POTUS wrong re [sic] so much," Rivera wrote. "But he's so right re gun violence."
Read his tweets on the shooting below:
Rivera has criticized the gun lobby after other mass shootings, including the Oct. 1 college shooting in Oregon that left ten dead.
"The mass murder at Umpqua again reveals we are a gun sick nation," Rivera said in a Facebook post. "All the 2nd Amendment bullshit aside, the gun lobby controls the country, and allows armed maniacal lunatics to roam free and kill."
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