Washington ? An audiotape from Osama bin Laden’s top deputy warning that the United States will pay dearly if it harms detainees at Guantanamo Bay is probably authentic, a CIA official said Monday.

The assessment was based on a technical analysis of the tape, which was first aired over the weekend, said the official, who spoke on condition of anonymity. The CIA can’t be certain but believes the voice on the tape was Ayman al-Zawahiri’s, the No. 2 official in bin Laden’s al-Qaida network, this official said.

The recording, obtained by the Arab satellite station Al-Arabiya, promises retaliation for Washington’s announcement that it could start putting the detainees on military trials that could result in death sentences.

“I swear by the almighty God … that Crusader America will pay dearly for any harm done to any of the Muslim prisoners it is holding,” the recording said.

The CIA said it could not determine when the tape was made. The reference to Guantanamo trials indicates it was made within the past month.

The recorded statement prompted warnings from Atty. Gen. John Ashcroft over the weekend of a “very real potential” for another al-Qaida attack. Ashcroft told “Fox News Sunday” that “al-Qaida wants to strike us.”

Ashcroft told ABC’s “This Week” that the tape “signals to us that the war is still under way, that al-Qaida still has the same intentions toward the United States that it did when it unleashed its savage attack” on Sept. 11, 2001.

The tape said all those who handed the prisoners to America or to any of its agents also will pay: “Let it be clear to those who conspire with America that America cannot defend itself, let alone defend others.”

The Arabic recording said every prisoner held by the infidels should know that his release is a “debt hanging from the neck” of every Muslim fighter, and “his brothers have not forgotten that they will avenge him from the new Crusaders.

“But we tell America one thing: what you have seen so far is nothing but the first skirmishes. The real battle hasn’t started yet.”

Al-Zawahiri’s whereabouts are not known. U.S. officials believe bin Laden, who was not mentioned in the audio tape, and al-Zawahiri are hiding in the wilderness near the Afghanistan-Pakistan frontier.