Cornell gave his plea before a U.S. district judge for the Southern District of Ohio.
Cornell allegedly planned to detonate pipe bombs at the Capitol and open fire on any employees and officials fleeing after the explosions, according to government documents at the time of his arrest several days before the 2015 State of the Union address.
According to the plea agreement, Cornell intended to kill officers and employees of the United States. He possessed two semi-automatic rifles and approximately 600 rounds of ammunition.
The FBI first noticed Cornell several months before his arrest after an informant notified the agency that Cornell was allegedly voicing support for violent “jihad” on Twitter under the alias “Raheel Mahrus Ubaydah,” according to the federal complaint at the time of his arrest.
"I believe that we should just wage jihad under our own orders and plan attacks and everything,” Cornell allegedly wrote in an online message to the informant in August, according to the FBI. “I believe we should meet up and make our own group in alliance with the Islamic State here and plan operations ourselves."
In the message, Cornell said that such attacks “already got a thumbs up” from radical cleric Anwar Awlaki “before his martyrdom.”
Awlaki was killed in a U.S. drone strike in 2011, but his online messages calling for attacks on the West live on.
After Cornell's arrest, he posted statements online that included a call for others to join him in violent jihad against the United States and its citizens on behalf of ISIS, according to the Justice Department.
He pleaded guilty to three counts overall and faces up to 20 years in prison for attempted murder of government employees; a maximum of 15 years for providing material support to a terrorist organization; and a mandatory five years for possession of a firearm in an attempt to commit a crime.
-Pierre Thomas, Jack Date, Mike Levine and Jack Cloherty contributed to this story.