I write to you under a terror alert issued Sunday by the Department of Homeland Security. The most difficult part of such an alert is not knowing precisely how to respond.

We are told of intelligence concerning al-Qaida plans for terrorist attacks against government and financial centers on U.S. soil: the International Monetary Fund and World Bank in the District of Columbia, Prudential Financial in Newark and Citigroup Center and the New York Stock Exchange in Manhattan.

Neither the government nor the public seems at ease with this alert. That’s what an alert is, by definition: uneasy.

I wish I could say the unease arose from fear alone. But no. It’s also a question of reliability.

Since the assaults on the Twin Towers in Manhattan and the Pentagon in Virginia, government officials repeatedly have been criticized for causing widespread unease as they issued terror alerts and vague information on threats when no immediate risk to the United States was at hand. The color security system has never gone below yellow or above orange. The level was raised to orange this week, an indication that the threat data were highly credible.

Authorities said on Sunday that the intelligence was unusually specific and detailed. It turns out, however, that it wasn’t new. The data were based mostly on plans developed back in 2000 or 2001. The latest information stemmed from an arrest in Pakistan.

And then there’s the little matter of the presidential election of Nov. 2. The reports apparently referred to the period running up to those elections. Some skeptics suggest that because the president is involved in a tight race with Sen. John Kerry, politics may be partly responsible for the new threat alert.

Faced with the increasing skepticism, Homeland Security Secretary Tom Ridge pointed out that al-Qaida “is very patient” and spends considerable time collecting information. “They collect, collect and collect, long before they strike,” Ridge warned. Another senior American official said that because of the specificity of the information, even though it is dated, “it is troubling.”

Authorities have increased security checks outside the New York Stock Exchange and at other financial institutions. In addition, they imposed new restrictions concerning underground parking, identification badges, and digital photos to keep track of people entering and exiting buildings.

“I’ve been placed on standby,” one police sergeant said this week. “I’m going to work today, but who knows when I’ll get back home. They’re treating this threat as if they expect something to happen soon. I guess it’s better to be safe than sorry.”

Some New Yorkers were surprisingly blase concerning the threats and the government’s reaction to them.

“There’s not much I can do but live my life,” Gerry Rogers said in an interview. “Sure, it all makes me nervous. But we all have to depend on the government to protect us. But every time somebody snaps his finger in New York, the noise makes me jump.”

None of us seem to have the answers — only a strong, if ill-defined feeling that at the bottom of it all is a spreading fear and persistent confusion.

“Every time I turn on the radio or TV, I hear about a new threat. I don’t know what to believe. One day the security color is yellow, the next it’s orange,” a man said in a street interview on television.

“Rumors are rampant. One minute the alert is on, the next they say it may be canceled,” one employee at J.P. Morgan on Wall Street said. “I’m only 55, but I’m going to retire and move to Florida. I can’t take this pressure any more. I had to walk across the Brooklyn Bridge during 9-11. Now this! How much can a person take?”

“That’s the way we live our lives these days,” she told a reporter. “This is the crazy world we’ve built.”