3. Close shot of sign on Dirksen building reading "Senate Offices Closed"
4. Police cars drive by the Senate office buildings
5. Wide shot of Senate floor
6. Wide shot press conference
7. SOUNDBITE: (English) Bill Frist (Republican from Tennessee) Senate Majority Leader:
"How active ? because people say is it active or not - preliminary tests are that it is active, how active we don't know, at this juncture. In terms of the size, shape, aerosolisation (if it can become airbourne), how sticky it is, additives, all that will come back over weeks."
8. Journalists at press conference
9. SOUNDBITE: (English) Bill Frist (Republican from Tennessee) Senate Majority Leader:
"We generally are very comfortable that everybody has been identified who was at risk and that the proper procedures were carried throughout. So, there's been no concern expressed that there are other people who might be involved."
10. Empty Senate hallways, shot through Dirksen front door, pull out to sign on door reading "Senate Offices Closed"
11. Set-up shot of David Heyman
12. SOUNDBITE: (English) David Heyman, Bioterror Expert:
"Ricin is not the most optimal agent for mass casualties. It's very good for specific targeted attacks against individuals, frankly. But to do damage on a mass casualty level, it's not at all an optimal agent. So I didn't think al-Qaida, or we're in for another big attack. But it is terrorising."
13. Dirksen office building sign
14. Exterior of Dirksen office building
15. Police officer talking to Senate staffers outside building
16. SOUNDBITE (English) David Heyman, Bioterror Expert:
"I suspect that it wasn't very well refined and so it probably didn't disperse around the atmosphere. It's quite likely that what we're learning here is that this was a crude attempt to scare Senator Frist, folks in Congress, perhaps even shut it down and get some attention, but unless we hear of other similar kinds of attacks, I think it's probably just a criminal act and not a terrorist attack."
17. Interior Senate
18. SOUNDBITE: (English) US Senate Minority Leader Tom Daschle, Democrat - South Dakota
"Terrorist attacks, criminal acts of this kind, will not stop the work of the Senate or the Congress because we have important work to be done."
19. Various of police cars outside Senate buildings
A white powder found in US Senate Majority Leader Bill Frist's office on Tuesday tested as the deadly poison ricin, forcing cancellation of most US Senate business in the second such scare from a lethal toxin to hit the US capital.
Between 40 and 50 Capitol employees were quarantined briefly and decontaminated, said Senate aides who spoke on condition of anonymity.
The number is considerably more than originally thought.
Although more tests were being conducted, Frist said he was positive the substance was ricin and officials of the Centers for Disease Control and Prevention said several of their tests identified it as ricin.
They said they were awaiting the results of additional confirmatory testing.
At the Senate office buildings, entrances were locked and many staffers were said to be working from home.
Capitol office buildings were eerily quiet, underscoring the sense that the area has essentially been under a terrorism threat since the attacks of September 11, 2001.
Senate leaders made a show of conducting business as usual on Tuesday, but canceled all votes that had been scheduled for the day.
As the Senate went into session, Senator Frist told his colleagues the ricin was certainly criminal in nature.
Bioterror expert David Heyman said ricin is not an most optimal agent for producing mass casualties, and he did not believe the incident is linked to al-Qaida or another foreign terror group.
Senate Minority Leader Tom Daschle, who had the deadly anthrax letters mailed to his office in 2001, said regardless of who is responsible, it was "an act of terrorism."
Officials said they were somewhat reassured by early afternoon on Tuesday because none of the people who were quarantined and decontaminated turned up sick.
President George W. Bush was briefed on the situation, and the administration established an interagency team to investigate what Frist told colleagues was a chilling crime.
A federal law enforcement official, speaking on condition of anonymity, said no threatening letter or note linked to the powder has been found.