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According to CBS:

• A Capitol Hill source identified the suspect as Amine El Khalifi, a Moroccan citizen who has lived in the United States for more than 12 years.

Fox News reported that the man had been the subject of an FBI investigation earlier this year.

• The man, in his 30s and of Moroccan descent, was nabbed following a lengthy investigation by the FBI, initiated after he expressed interest in conducting an attack. It's unclear how the FBI learned of his aspirations.
• The man thought undercover FBI agents assisting him in his plot were associates of Al Qaeda.
  
• A short time earlier, he had been praying at a mosque in the Washington area. His destination was Capitol Hill.

USA Today:
A federal law enforcement official, who was not authorized to give public comments, told USA Today that  "the suspect was the target of an undercover investigation for about two months and that the public was never in any danger."

The following statement was also issued by U.S. Capitol Police, reported NBC Washington:

"On Friday, February 17, 2012, members of the U.S. Capitol Police and the FBI arrested an individual in the area of the U.S. Capitol.  This arrest was the culmination of a lengthy and extensive operation during which the individual was closely and carefully monitored.  The U.S. Capitol Police was intimately involved in the investigation for the duration of the operation.  At no time was the public or Congressional community in any danger."