As bombing and missile attacks continue between Israel and Hezbollah, Israeli officials have warned their diplomats around the world about the threat of Hezbollah sleeper cells.
There is no credible intelligence pointing to an imminent Hezbollah attack on the United States, but sources say to ABC News that the group is capable of striking here.
The video released Thursday of a statement of support for Hezbollah by al Qaeda's No. 2, Ayman Al-Zawahiri, only added to the unease.
The spokesman for Hezbollah in Iran also has said the group supports its Lebanese counterparts, though the two groups are only loosely affiliated.
"We have 2,000 volunteers. … If America wants to ignite WWIII, we welcome it," said the Iranian Hezbollah spokesman.
Harry Skip Brandon, the former chief of the FBI Counterterrorism Division, said the United States was watching the group closely.
"All of U.S. law enforcement and intelligence are looking real hard now at the threat posed by Hezbollah," Brandon said.
This week, the FBI sent a notice to 18,000 police agencies warning them to remain vigilant about Hezbollah.
ABC News has learned that Hezbollah has maintained a sizeable presence in the United States.
In fact, at one point last year, the FBI had more than 200 active cases involving suspected Hezbollah members.
Counterterrorism analysts, however, are split over the likelihood of a Hezbollah terror strike here.
"I think it's a major threat right now," Brandon said.
Jack Cloonan, a former FBI counterterrorism agent, said, "The more likely scenario will be that Hezbollah will target a U.S. facility overseas."
If Hezbollah tries to activate terror cells in the United States, there could be catastrophic results.
In Beirut, Lebanon, in 1983, more than 200 people were killed in a Hezbollah suicide truck bombing of a U.S. Marine Corps barracks.
In 1996, Saudi Hezbollah was accused of participating in the bombing of the Khobar Towers in Saudi Arabia that killed 19 Americans and wounded more than 300.
Before Sept. 11, 2001, in fact, some U.S. officials were more concerned about Hezbollah than al Qaeda.