about another investigation right now, bring you up to date on another dangerous mystery and drama. This one in washington, d.C. Today parts of the cap pal building were evacuated after a suspicious... See More
about another investigation right now, bring you up to date on another dangerous mystery and drama. This one in washington, d.C. Today parts of the cap pal building were evacuated after a suspicious package was found. night, a terror investigation is under way because of a letter sent to a senator testing positive for a type of poison called ricin. Today, a second letter found addressed to the president. It's a poison that can kill within 36 hours. There's no antidote. Abc's jim avila with this investigation. Reporter: Hazmat suited inspectors today gathering evidence perhaps linked to a suspected poison laced letter addressed to the president of the united states and a mississippi senator. While on capitol hill, mail delivery is stopped and staff warned not to open unsealed envelopes. All staff are directed to avoid this area until further notice. Reporter: The fear --ricin, easily made from castor beans by would-be-bio-terrorists. If you ingest enough of it, it could be very damaging and it could kill you. Reporter: Two letters, one addressed to president obama, another to mississippi senator, roger wicker, showed initial positive results for ricin in not always reliable field tests. The letters never reached the white house or the capitol. The mail sent here is screened, and that these tests are undertaken at remote sites to mitigate the risk. Reporter: The first addressed to senator wicker was stopped in suburban maryland, 10 miles for its intended target on capitol hill. The second letter was intercepted 6.5 miles away on a military base on the outskirts of d.C. A third post office sorting facility suffered possible contamination to its filtering system from an unknown source -- some ten miles from downtown washington. Those protections, including air sensors at screening centers, set up after the anthrax attacks in 2001 that killed five americans and injured 17 others in this country's worst ever bio-terrorist attack, just after 9/11. Today's apparent attempt, so close to another incident of domestic terror, an eerie reminder of those frightening days.Now the fbi I investigating the suspected poison in the letters both of which contain the same weird message, but so far, no suspects. And we want to bring in richard clark, an abc news consultant with the national counterterrorism adviser at the white house under two administrations. Can you trace this envelope containing ricin? We may be able to. Ricin leaves a trail of breadcrumbs, if you will. Authorities are going to the post office in memphis where it was post marked, trying to find it there. Little r in 2001, w were able track it back to a particular mailbox in new jersey with the anthrax. It may be possible to do that with the ricin. So no reason to believe there's a connection between that and boston? There's no evidence and my gut says it's a coincidence. Let's turn to boston. You've told us there were a couple lucky events on this awful day. The two main streams of evidence, putting the bomb back together again, good news there because the bomb didn't detonate very well, there's a lot of it left. So a lot of it has already been put back together. The other stream of evidence is the surveillance tape, and there's good news there apparently, that they have some faces. So we may be in one of those cases that gets cracked quickly. Too early to tell yet, but we may know very soon. In terms of putting it together, you reminded us they put back together the plane at lockerbie, pan am 103. It was a 747 that blew off at altitude, spread over a hundred square miles. The fbi put that plane back together, just as they are today putting the bombs back together. All right. I know you'll be standing by throughout the evening. Thanks.
This transcript has been automatically generated and may not be 100% accurate.