By Robert Windrem, NBC News Producer

Al-Qaida deputy Ayman al Zawahiri appeared in a new propaganda video late last month, lamenting the killing of a high-ranking member of the terror network. Not that long ago, analysts at the CIA would have combed the video for hidden messages--possible “go signals” for terrorist attacks.

Was there something sinister inserted in the Koranic verse at the beginning of the tape, they might wonder, or did the video itself mask an embedded message? Analysts still do textual and video analysis of Al Qaida statements, but the likelihood that messages were secreted in the video is not as high as once thought.

Why? Credit cyber-security advances. U.S. intelligence officials and other terrorism experts say that Al Qaida and related jihadist organizations have mastered cyber security in ways that many terrorism analysts find impressive, vexing and troubling.

By Jim Popkin, NBC News Senior Investigative Producer

In a bulletin released Friday to U.S. law enforcement officials, the Transportation Security Administration (TSA) is warning of “continued strong terrorist interest” in targeting mass transit systems in the U.S. The 10-page threat assessment, labeled “Unclassified/For Official Use Only” and obtained by NBC News, cautions that the “U.S. mass transit and passenger rail systems are vulnerable to terrorist attacks because they are accessible to large numbers of the public and are notoriously difficult to secure.”  Previous rail attacks in Madrid, London and Mumbai “could inspire terrorists to conduct similar attacks in the United States,” the report adds.

However, the authors of the intelligence analysis make clear that there are no known, immediate dangers. “At this time, there is no credible intelligence regarding specific plans by any extremist groups or individuals to perpetrate an act of terrorism against the U.S. mass transit system,” they write.

By Robert Windrem and Garrett Haake, NBC News

The State Department has been steadily increasing the number of visas granted to students and visitors from three Islamic nations -nations with connections to the Sept. 11 attacks and to al-Qaida, according to an NBC News survey of U.S. visa data.

Many counterterrorism experts welcome the increase in visas to Saudi Arabia, Egypt and Pakistan, arguing that exposure to American culture outweighs any possible risk from prospective terrorists.

By Aram Roston, NBC News Producer

One of the only Americans to have met Imad Mugniyeh, the terrorist leader killed in Syria, tells NBC News that the Hezbollah militant burned with "hatred" for the United States.

Kurt Carlson, of Illinois, was one of the hostages onboard TWA Flight 847 when it was hijacked in 1985 during a flight from Athens to Rome. In Beirut, the hijackers shot U.S. Navy diver Robert Stetham, a passenger on the plane, and dumped his body on the runway.

By Robert Windrem, NBC News Producer

U.S. counter-terrorism officials tell NBC News that Hezbollah commander Imad Mugniyeh, killed today in a car-bomb attack in Syria, was the most highly sought terrorist after al-Qaida leaders Osama Bin Laden and Ayman al-Zawahiri. The reason, they say, is simple: Mugniyeh was responsible for more American deaths than anyone other than the al-Qaida terrorists.

“Other than a Bin Laden, no one is more dangerous than this guy,” a senior western counterterrorism official told NBC News.  “He has a huge rap sheet, and don’t believe this canard that he had ‘retired’.  He was very active and still dangerous.  He was the architect of every Hezbollah attack the last quarter century.”

By Carol Grisanti and Mushtaq Yusufzai, NBC News, Pakistan

The Taliban may have scored a big propaganda victory again. They appear to have kidnapped the Pakistani Ambassador to Afghanistan, Tariq Azizuddin, Pakistani officials tell NBC News.

The Ambassador has been missing for nearly two days, and officials presume that he was kidnapped in Khyber, one of the seven lawless tribal areas along the Pakistan-Afghanistan border. His car was recovered in Zakhakhel, a village in the Tirah Valley of the Khyber Agency, where the mountains are high and the roads are few. Only the locals and the professional smugglers know the well-worn footpaths that lead through the valley and over the mountains into Afghanistan.

By James Popkin, NBC News Senior Investigative Correspondent

The U.S. Department of Homeland Security is reminding law-enforcement officials of the increased use of female suicide bombers worldwide, and warning that women terrorists might hide explosives in devices “that mimic the look of a pregnant woman.”

The unclassified DHS threat assessment, released Monday and obtained by NBC News, shows photographs of “pregnancy prosthetics,” hollowed-out devices that could hold explosive devices. The report says “female suicide bombers have used devices that make them appear pregnant to hide explosive devices.”