"Why does a teenager legally have an AR-15? Somewhere along the line, these guys forgot they work for us, not the NRA. Us," he said during Thursday night's monologue. "And this time we're not going to allow you to bow your head in prayer for two weeks until you get the all-clear and move on to the next thing."
ABC News reports that 19-year-old Nikolas Cruz, who has been charged with 17 counts of premeditated murder in Wednesday's shooting in Parkland, Florida, obtained his gun legally. Law enforcement sources said that the paperwork was in order and appropriate procedures were followed when the AR-15 was purchased, which was within the past year.
In his monologue, Kimmel called for action.
"Children are being murdered," he said at one point, pausing to collect himself.
"Do something now!" an audience member shouted.
"Do something," Kimmel agreed. "We still haven't even talked about it."
Kimmel also referenced H.J. Res. 40. This ended the requirement to enter names of people who receive mental health benefits into the National Instant Criminal Background Check System, which is used for background checks when guns are purchased.
On Thursday, Trump referenced mental illness in a tweet about the Parkland shooting.
So many signs that the Florida shooter was mentally disturbed, even expelled from school for bad and erratic behavior. Neighbors and classmates knew he was a big problem. Must always report such instances to authorities, again and again!— Donald J. Trump (@realDonaldTrump) February 15, 2018
"So I agree, this is a mental illness issue," Kimmel continued, "because if you don't think we need to do something about it, you're obviously mentally ill."
Kimmel also directly addressed those who continue to say that it's "too soon" to talk about reform.
"Don't you dare let anyone say it's too soon to be talking about it, because you said it after Vegas, you said it after Sandy Hook, you say that after every one of these eight now fatal school shootings we had in this country this year," he said.
This number seems to be a reference to a study by the group Everytown for Gun Safety, according to ABC News, with the correct statistic being eight school shootings that were intentional and resulted in injury or death.
In October, Kimmel also spoke out against the Las Vegas shooting in an emotional monologue viewed online by millions.
"With all due respect, your thoughts and your prayers are insufficient," he said at the time. "We have a major problem with gun violence in the county and I guess they don't care."
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ABC News contributed to this report.