A U.S. Senate office building and two other privately owned properties on Capitol Hill were recently identified as possible targets for a terrorist attack, prompting a visit from FBI agents to each location, according to government and law enforcement sources.

The warnings were delivered Feb. 9 -- two days after the terror alert was raised to code orange -- and were based on information that was soon discredited. But the specific nature of the threats, which included the three addresses on Capitol Hill and also mentioned federal buildings in Richmond and Norfolk, set off the delicate process of informing property owners and managers.

What happened next illustrates the difficult questions that arise when a specific threat is delivered, regardless of its reliability. Does the owner of an office building notify tenants? Should business be curtailed until the threat is deemed over? Should employees be relocated? What is a prudent precaution in an age of vague and seemingly constant threats?

With the nation under Code Orange since Feb. 7, the second highest possible level of terror alert, the predicament confronting owners and managers in such cities as Washington and New York is real. FBI agents told the managers of two of the buildings that they were carrying out a policy of disclosing specific threats even as they took pains to describe them as "low credibility."

"This is the tip of iceberg as to how we are going to deal with these issues and what are the proper policy responses," said David Siegrist, research fellow and director of studies for countering biological terrorism at the Potomac Institute for Policy Studies in Arlington. "If there's specific critical information, you pass on the information. Certainly if you don't pass on the information and, God forbid, there's an attack, the survivors could expect to sue and reasonably find you liable."

In the case of the three buildings, the threat produced three different responses. At the Senate, leaders were notified, but no warning was delivered to the majority of the staffers or workers. In the second building, the owner followed the advice of the FBI and promptly notified tenants in a hand-delivered memo.

In the third building, the property manager said the FBI told security workers to be on heightened alert but provided no specifics. One law enforcement source said FBI agents were concerned because media outlets based in the building might create more alarm than was warranted if they learned of the threat.

Law enforcement sources said the threats prompted more police patrols and other security measures on Capitol Hill and in Richmond and Norfolk. "We took appropriate, direct, affirmative steps to address the nature of the threat, as we do with any threats," said Van Harp, head of the FBI Washington field office, who declined to comment on specifics of the case.

The visits by agents from the FBI's Joint Terrorism Task Force were unusual because they were prompted by threats to specific office buildings as opposed to symbolic landmarks, such as the U.S. Capitol or White House. The information about the threats was provided by a confidential informant for the government and was among many factors that resulted in raising the terror alert to Code Orange, according to two law enforcement sources.

The other two, privately owned buildings have the kind of tenants common on Capitol Hill: political action committees, lobbyists, news media outlets and trade associations. "None of them were buildings where you would think a larger group of tourists would even know about them," one government official said. "It was unusual."

Steven L. Cymrot, the owner of one of the buildings, said FBI agents visited him on a Sunday afternoon, Feb. 9, and told him about the threat. He said he passed the news along without much agonizing.

"You're talking about a neighborhood where there are Humvees with missiles going by, droning sounds at night overhead, Cymrot said. "Nah, I wouldn't say this was anything out of the ordinary."

At the FBI's suggestion, Cymrot sent a memo to tenants the next morning. The notice, still posted in some offices, states in part: "It is the current policy of the FBI to give notice of all threats regardless of credibility. We felt it was our duty to do the same, despite the fact that the task force did not feel this to be a serious threat."

Cymrot said he had a fleeting vision of tenants moving out. "My question is, what's the purpose of this?" he said. "If you're telling me there's no action I should be taking in response, then are you doing anything more than covering yourself in a later lawsuit?"

But only one tenant responded with any concern. Others shook off the warning, seemingly hardened by Washington's constant state of alerts. "It really didn't phase me, another threat on D.C.," said Frank Rizzo, a tenant in the building. "You can't let terrorism paralyze you. I went about my business."

"I'm not much of an alarmist. If your time's up, your time's up, you know?" said Patricia McNaughton, director of operations for a nonprofit organization.

The situation unfolded differently at the third building, which houses trade associations, bureaus for a news organizations and lobbyists. Tim Richardson, senior property manager for Trammell Crow Co., which manages the building and many other properties, said FBI agents and U.S. Capitol Police officers who visited Feb. 9 to review standard security protocols seemed to pay particular attention to one area and then left. One person familiar with the situation said agents appeared less concerned when they were told who was occupying a particular suite.

Morris J. Amitay, a tenant at the building, said he had no problem with tenants not being notified. "We're all pretty sophisticated, and we realize how close we are to the Capitol," Amitay said. "These threat notices are so vague and general I don't think people pay much attention."

Capitol Police Chief Terrance W. Gainer referred specific questions about the threats to the FBI. "We're getting good information from the FBI and people working at the CIA," Gainer said. "I'm very comfortable about how we received it, processed it and the outcome."

At the Senate, according to two sources, Sergeant-at-Arms Alfonso E. Lenhardt informed Majority Leader Bill Frist (R-Tenn.), whose spokesman declined to comment. A spokeswoman for Minority Leader Thomas A. Daschle (D-S.D.) also declined to comment. But aides contacted at other Senate offices housed in the building said they were unaware of the threat.

Two law enforcement sources said authorities determined that the room number cited by the FBI had been vacated by a senator no longer in office and was being renovated.

Passing along specific threats, especially if the information is thin, creates its own problems. W. Shaun Pharr, vice president of government affairs for the Apartment and Office Building Association of Metropolitan Washington, said such warnings could panic tenants, harm business, reduce the marketability of properties and affect insurance rates.

"Generally, the stakes and the potential consequences [of an attack] are so high that most property owners and managers are not going to want to be in denial of a threat," Pharr said. "But the flip side of that is: If you know something, are you obliged to take certain actions in response?"

Michael O'Hanlon, senior fellow at the Brookings Institution, said the onus should be on the government, which should suggest concrete steps businesses should take and to explain clearly the trustworthiness of information.

"That is the toughest kind of call in this business -- when you have some very weak information, but specific information," O'Hanlon said. "When you have weak information, we should assume it's probably wrong. As long as that's conveyed, it's okay. You have to have an extraordinary and diplomatic touch."

To security experts, such concerns are secondary. Alexander Tabb, associate managing director of security services for Kroll Inc., a risk consulting firm, said it's important not to deny the existence of a threat. Still, he admitted, there are no easy answers. "It's not often you get Uncle Sam knocking at the door, saying, 'There's someone who may want to kill you, and we have no idea if it's credible or not,' " he said. "That's why companies struggle with decisions."

Staff writer Susan Schmidt and staff news researchers Julie Tate and Madonna Lebling contributed to this report.