The al-Qaida threats that led to the closure of almost two dozen U.S. diplomatic posts Sunday is the most “serious threat that I’ve seen in the last several years,” Sen. Saxby Chambliss tells NBC News. The ranking member of the Senate Intelligence Committee added that the “chatter” among suspected terrorists is "very reminiscent of what we saw pre-9/11.” House Homeland Security Chairman Mike McCaul agreed with that assessment, telling CBS News that what led to the closing of 22 embassies and consulates in 17 countries in the Middle East, North Africa, and South Asia is “one of the most specific and credible threats I've seen, perhaps since 9/11.”

For his part, Rep. Dutch Ruppersberger, the ranking member of the House Intelligence Committee said “high level people from al-Qaida in the Arabian Peninsula are talking about a major attack.” The Democrat from Maryland told ABC News Sunday that “it’s a very credible threat.”

A “senior U.S. official” tells ABC News that intelligence officials intercepted communications in which al-Qaida operatives could be heard talking about an upcoming attack. The alleged terrorists talked about how the attack is “going to be big” and “strategically significant,” according to the official who added that what was particularly “alarming” was the “confidence they showed … and the air of certainty.” Still, the official emphasized that the target of the planned attack remains a mystery: “We do not know whether they mean an embassy, an airbase, an aircraft, trains.”

Sunday is a significant day in the Muslim calendar because it marks the 27th day of Ramadan and is known as the “Night of Power,” when “the first verses of the Koran were revealed to the Prophet Mohammed,” explains CNN’s Peter Bergen. Al-Qaida’s would-be martyrs see the date “as a particularly auspicious day to die,” adds Bergen.