Story highlights A "threat stream" from al Qaeda groups has evolved during the past six months

The threats target Europe as well as the United States

Official to CNN: So far, "we do not see operational cells of al Qaeda inside the United States"

Al Qaeda in the Arabian Peninsula, based in Yemen, is of particular concern

A series of al Qaeda-based threats to attack American and Western targets in Europe, as well as threats to launch attacks inside the United States, has caused significant concern inside the U.S. intelligence community, CNN has learned.

Officials are trying to determine the extent to which the threats may be linked and determine what it may mean about the strength of al Qaeda in several countries. While the "threat stream" has evolved during the past six months, according to a senior U.S. official, none of the threats has been corroborated.

The official said the threats appear to detail "a lot of activity where intelligence suggests there are operational cells," but so far, "we do not see operational cells of al Qaeda inside the United States," the official said, although he emphasized it could not be ruled out.

U.S. officials as well as analysts have long said they believe al Qaeda is focused on attacking outside the United States because of the difficulty in penetrating U.S. homeland security. But these latest threat streams suggest domestic U.S. targets also continue to be looked at by al Qaeda.

Worry No. 1: Al Qaeda in the Arabian Peninsula

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One of the biggest concerns right now remains efforts by Yemen-based al Qaeda in the Arabian Peninsula to attack inside Yemen, in Europe and inside the United States.

The U.S. Embassy, which was closed several weeks ago, is now expected to remain closed for several more days because of a "credible threat stream," a second senior U.S. official told CNN.

While it's believed AQAP is mainly focused on targeting inside of Yemen, officials say an attempt to attack inside the United States or to recruit others to carry out such an attack cannot be ruled out.

Worry No. 2: Core group in Pakistan

Separately, still another official confirms to CNN that the core al Qaeda group in Pakistan is also seeking to place operatives in places to attack U.S. targets overseas and domestically. U.S. officials confirm an operative who uses the code name Abdullah al-Shami, who is living in Pakistan, may now be heading that group's efforts to plan external attacks.

The New York Times first published al-Shami's name in February and noted that U.S. intelligence believes he was born in the United States but moved to the Middle East as a child.

The New York Times reported that as an American-born citizen, al-Shami could be a target for attack by a U.S. drone. U.S. officials refused to comment, but a private sector analyst who has followed the matter in detail told CNN that killing al-Shami had been discussed in the past.

If al-Shami indeed is activity pursuing attacks against the U.S., it would indicate that the core al Qaeda group in Pakistan might be resurgent in its capabilities.

Worry No. 3: Spillover effects from Syria

The third threat stream continues to come from Syria. U.S. officials tell CNN they believe 70 or more Americans have traveled to Syria to fight, and many may have returned home along with other fighters with Western passports.

Over the past several months, the U.S. has sought to closely track all Americans traveling to Syria.

Earlier this month, the Treasury Department imposed sanctions on an alleged al Qaeda operative, Abd Al-Rahman Muhammad Zafir Al-Dubaysi Al-Juhni.

Treasury said that in 2013, he was part of a group of senior al-Qaeda members in Syria formed to conduct external operations against Western targets.

It's also believed Al-Juhni was in Pakistan at some point, putting him in a position to have interacted with al-Shami.