Washington ? The intelligence that led to this week’s heightened terrorism alert came in part from a senior al-Qaida operative in British custody who disclosed that the terrorist network was continuing to plan an attack in New York, possibly around the time of the Republican National Convention, U.S. officials said Wednesday.

The officials said that warning was received simultaneously late last week with the discovery of al-Qaida computer files in Pakistan showing would-be terrorists had conducted surveillance of five buildings in New York, Washington and New Jersey beginning in 2000 and 2001.

The Bush administration is under intense pressure to justify its decision Sunday to raise the terrorism-alert level for financial districts in the three locales. Critics have decried the decision as unnecessary and even politically motivated.

Making their case, White House officials said Wednesday that, in addition to the computer files, top policy-makers late last week “received intelligence about a separate, current threat to the U.S.,” in the words of one U.S. official.

The information “represented another serious threat to New York,” said the official, who spoke on condition of anonymity. The White House declined to describe the information further, citing security concerns.

“I can’t go further into it because it could compromise some ongoing (counterterrorism) operations,” White House spokesman Scott McClellan said.

A former U.S. official with close ties to the intelligence community said he was told the warning was relayed by the operative said to be in British custody, who’s cooperating with investigators.

The operative, whose existence was first reported Monday by Knight Ridder, told authorities that al-Qaida was planning an attack 60 days before the Nov. 2 presidential election, said the former official, who also spoke on condition that he not be identified.

That would coincide with the Republican National Convention, where President Bush is to be nominated.

New York City Police Emergency Services officers continue their patrols outside the New York Stock Exchange on Wall Street. Authorities said Wednesday that a senior al-Qaida operative in British custody disclosed that the terrorist network was continuing to plan an attack in New York, possibly around the time of the Republican National Convention.

The circumstances of the operative’s detention remain unclear.

British officials and a CIA spokesman declined comment.

None of the evidence found in Pakistan indicated that preparations for an attack on the buildings in New York, Washington and New Jersey went beyond the planning stage, he said.

Larry Johnson, a former top CIA and State Department counterterrorism official, warned that al-Qaida could be engaged in a deception campaign aimed at confusing U.S. authorities about its plans and fatiguing American defenses by forcing security measures to change constantly. “I think that’s where we’re headed,” he said.

Administration officials said they had three separate indicators of a possible attack: general intelligence showing that al-Qaida wants to strike the United States during the election season; the warning aimed primarily at New York; and the captured computer files.