The FBI has arrested an alleged supporter of the Islamic State of Iraq and the Levant (ISIL) over a suspected plot to attack the US Capitol building and kill government officials.

Christopher Cornell was arrested and charged on Wednesday with attempting to kill a US government officer and possession of a firearm.

According to court documents, the FBI said Cornell researched the construction of pipe bombs, purchased a gun, 600 rounds of ammunition and made plans to travel to Washington to carry out the attack.

Al Jazeera's Patty Culhane, reporting from Washington, said Cornell started a Twitter profile sympathetic to ISIL, made contact with the FBI informant through the social media site and then developed plans to attack the capital building with pipe bombs.

"It's not clear whether the plot was directed from overseas," our correspondent said.

"Cornell gives conflicting statements according to the court documents, at one point claiming that al-Qaeda in the Arabian Peninsula [AQAP] and Anwar al-Awlaki had signed off on the attack, and then claiming that ISIL supports it."

The suspect's father, John Cornell, told reporters his son "was coerced into a lot of this".

In a November meeting, Cornell told the FBI informant he considered members of Congress to be his enemies. He reportedly met the informant twice late last year. 

Source: Al Jazeera and agencies