The FBI arrested Christopher Lee Cornell of Cincinnati, charging him with buying weapons to carry out a terrorist attack on Washington, D.C. Cornell, 20, was monitored by federal agents who say he used Twitter to express support for the extremist group Islamic State as well as "violent jihad."

The arrest warrant for Cornell, who authorities say was known online as Raheel Mahrus Ubaydah, says that he "purchased and possessed firearms in furtherance of a plan to shoot and kill United States Government officers and employees."

The FBI says the case dates to messages Cornell posted online last summer.

NPR's Carrie Johnson reports:

"Federal authorities say Christopher Lee Cornell operated a Twitter account where he posted support for violent jihad. For months, the FBI used a confidential source to get close to Cornell, who allegedly said he wanted to hatch a plot inside the U.S.

"The government says he wanted to attack the Capitol building by detonating pipe bombs and then shooting federal workers. Law enforcement agents arrested Cornell after he bought weapons."

The FBI says Cornell had taken possession of two "Armalite Inc., Model M-15, 5.56mm, semi-automatic rifles ... and approximately 600 rounds of ammunition."

The criminal complaint filed against Cornell says that the government's confidential source cooperated with the authorities in the hopes of getting "favorable treatment with respect to his criminal exposure on an unrelated case."

In addition to the tweets, the complaint says Cornell used instant messages to discuss his plans, quoting him as telling the informant, "I believe we should meet up and make our own group in alliance with the Islamic State here and plan operations ourselves."

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