(CBS/AP) The CIA has determined that the voice purported to be Osama bin Laden's on a recent videotape is probably authentic, an agency official said Friday.

The official, who spoke on the condition of anonymity, said further technical analysis had led the agency's experts to the conclusion.

The official, discussing the situation on grounds of anonymity, cautioned that analysts weren't completely certain. The CIA official also noted that there are no references in the recording of bin Laden that would indicate when that recording was made.

Earlier, an official said the intelligence agency had certified that a section of the audio on the new al Qaeda videotape was an actual recording of Osama bin Laden's chief deputy.

President Bush said Thursday that while the tape was still being analyzed, it "reminds us that the war on terror goes on."

"His rhetoric is trying to intimidate and create fear," Mr. Bush said after a trip to Walter Reed Army Medical Center, where he visited troops wounded in the war in Iraq. "He's not going to intimidate America. We are at war because of what he and his fellow killers decided to do two years ago today. We will stay the course until we have achieved our objective and dismantled the terrorist organizations."

Bin Laden, considered the man behind the Sept. 11, 2001 terrorist attack on the United States, was an early focus in the president's "war on terrorism."

After U.S. troops pinned him down in the Tora Bora mountains in December 2001, many American officials believed he is dead. But a number of messages from him have emerged since, and now he is believed to be alive and in the rugged, lawless region on Pakistan's border.

The videotape was broadcast Wednesday over the airwaves of the Arab al-Jazeera television network. It contained a pair of voiceovers, one purportedly from bin Laden, and the other from deputy Ayman al-Zawahri, an official said.

Initially, the technical analysis of the section with bin Laden's voice was inconclusive, that person said.

In the recording, Al-Zawahri mentioned the war in Iraq, which suggested that the tape might have been recorded sometime since the fighting started in March. The voice purported to be bin Laden's speaks of some of the Sept. 11 hijackers but makes no reference to more recent events.

On Thursday, As the nation remembered the deadliest terrorist attack ever on its shores, the State Department warned that it is seeing "increasing indications that al Qaeda is preparing to strike U.S. interests abroad."

"We expect al Qaeda will strive for new attacks that will be more devastating than the September 11 attack, possibly involving nonconventional weapons such as chemical or biological agents," the warning read. "We also cannot rule out the potential for al Qaeda to attempt a second catastrophic attack within the U. S."

However, the warning did not cite new intelligence or any specific plans or targets.

It was not accompanied by any rise in the terrorist alert level, which remains at yellow, or "elevated." On the CBS News Early Show, Homeland Security Secretary Tom Ridge indicated the terror alert status was at an appropriate level.

CBS News National Security Correspondent David Martin reports U.S. officials say the terrorist threat was actually higher last year, at the first anniversary of the 2001 terrorist attacks. The national terror alert status was raised to "high" for that anniversary.