Bomb plot sting's troubling twist

>>> claiming tonight it has stopped a lone wolf terrorist attack on u.s. soil as part of a sting operation set up by the government. in any kind of a sting where the government trails a suspect then swoops in and arrests him before the crime, we never get to find out what would have happened in the end. that is exactly the case tonight in our nation's capital where the fbi has arrested a man in such a sting operation , who they say intended to go to the u.s. capitol building and blow himself up. it's where we begin tonight with the reporting of our justice correspondent pete williams . pete, good evening.

>> reporter: federal agents say what makes this sting operation different is that the man they arrested was willing to carry out a suicide attack wearing a bomb, with one of his targets possibly the capitol visitor center right behind me. they say he never actually had any explosives. police and fbi agents combed through the home of a 29-year-old man originally from morocco in the washington suburb of alexandria, virginia. he's accused of planning to wear a suicide vest and set it off at the u.s. capitol . he told investigators he hoped to kill at least 30 people. fbi agents had him under close surveillance since early december. first he told them he wanted to attack jewish synagogues in the area or a restaurant or u.s. military facilities. then he decided his target would be the capitol building , perhaps its new underground visitors center likely to be crowded with tourists on a week day afternoon.

>> we've seen suspect whose wanted to detonate a device but do it from a distance. they didn't want to be suicide bombers . in this case, he wanted to be the martyr

>> reporter: a month ago they went to a quarry in west virginia and set off explosives in a practice run. around noon today in a parking garage near the capitol, the fbi says the undercover agents gave him a vest loaded with way thought were explosives and a gun which turned out to be nopt rabble. he was arrested as he walked toward the capitol.

>> apparently his intention was to shoot capitol police outside the capitol and at the entrance, then go in and set off the vest. there are a large number of tourists there today.

>> reporter: the fbi says it first became interested in el kalifi after a year ago from receiving a tip. it's impossible to know if he could pull it off on his own, but disturbing how far he was willing to go.

>> this kind of single person, so-called aspirational terrorist event is what they've been worried about for years. i guess it's a matter of staying ahead of them.

>> reporter: right. of course, what they say is that in cases like this, it's people who will go beyond merely making statements. yes, they became interested when he was meeting with other people talking about this. they say it's the series of steps he kept taking, escalating every step of the way how far he was willing to go that made them decide now was the time to make the arrest.

>> pete williams from the capitol and the story he's been reporting on all day. thanks.