Media reports indicate that U.S. officials are concerned another tack on the U.S. is being planned by Al Qaeda in Yemen.

According to NBC's Pete Williams, intelligence officials are identifying signals that an attack is in the works that are similar to communication missed before the attempted Christmas Day bombing of a Detroit-bound airliner.

It's not clear whether anyone has determined the nature of the possible attack or where it might originate.

According to U.S. Rep. Frank Wolf (R-VA), the State Department believes there are 55,000 Americans visiting or living in Yemen. In a letter to Deputy National Security Advisor John Brennan Tuesday, Wolf asked how the government is assessing the threat posed by "the possible radicalization of those who can fly back to the U.S. with American passports? This is especially troubling in light of the fact that the Yemeni government does not control large portions of the country outside the capital city."

Wolf's letter asks a series of Yemen-related questions, noting that terror plots against the United States involve people who were radicalized there, or were influenced by American-born cleric Anwar Al-Awlaki.

Among them:

Whether six Guantanamo detainees sent to Yemen in December are in government custody or whether reports that they have been released to their families are true.

Whether would-be underwear bomber Umar Farouk Abdulmutallab told officials he trained with other Al Qaeda operatives who were plotting similar attacks.

A series of questions about Abdulmutallab's interrogation and the decision to charge him criminally, rather than as an enemy combatant.

Wolf also wants to have the chairmen of the 9/11 Commission identify which of their recommendations remain unfulfilled. See his letter to Brennan here.