U.S. officials believe that as many as three dozen Americans who converted to Islam while in prison in the United States have traveled to Yemen over the last year, possibly to be trained by Al Qaeda, according to a Senate report.

The findings have alarmed U.S. counter-terrorism officials, who think that Al Qaeda has expanded its recruitment efforts in Yemen “to attract nontraditional followers” capable of carrying out more ambitious operations.

The report by the Senate Foreign Relations Committee underscores the growing anxiety in the United States about the Al Qaeda offshoot, Al Qaeda in the Arabian Peninsula, which has claimed responsibility for orchestrating the suspected attempted suicide bombing of a U.S. jetliner bound for Detroit from Amsterdam on Christmas Day.

“The Christmas Day plot was a nearly catastrophic illustration of a significant new threat from a network previously regarded as a regional danger, rather than an international one,” the report concluded. It warns of “growing evidence of attempts by Al Qaeda to recruit American residents and citizens in Yemen, Somalia and within the United States.”

The report was released in advance of a hearing that the committee is scheduled to conduct today on Al Qaeda’s resurgence in Yemen.

“As many as 36 American ex-convicts arrived in Yemen in the past year, ostensibly to study Arabic,” the document said. Some of those Americans “had disappeared and are suspected of having gone to Al Qaeda training camps in ungoverned portions of the impoverished country.”

The estimate was attributed to interviews that committee staff members conducted with U.S. intelligence and law enforcement officials in Yemen and other countries in the Middle East in December.

Fears about the Americans in Yemen are portrayed as part of a larger pattern of U.S. citizens or residents being drawn overseas to organizations with ties to Al Qaeda, raising concerns that they may be able to reenter the United States more easily than foreigners after undergoing terrorist training.

The document refers to “two dozen Americans of Somali origin who disappeared in recent months” from St. Paul, Minn., and are widely suspected of fighting alongside Shabab, a militant group in Somalia linked to Al Qaeda.

In addition to ex-convicts, the report makes mention of as many as a dozen other U.S. citizens who have traveled to Yemen after marrying Muslim women and converting to Islam.

U.S. intelligence officials declined to comment on the report, which did not identify its sources by name or agency.

CIA-operated Predator aircraft have carried out strikes on Al Qaeda targets in Yemen in recent weeks.

The Senate panel conducted the interviews before the Christmas Day jetliner incident, which was thwarted when fellow passengers subdued the suspect after explosives he allegedly had smuggled on board in his underwear failed to detonate.

The 23-year-old suspect, Umar Farouk Abdulmutallab, has been indicted by a federal grand jury in Michigan on charges of attempted murder and the attempted use of a weapon of mass destruction.

U.S. counter-terrorism officials have expressed concern that Abdulmutallab, who is the son of a prominent Nigerian banker and had a visa to enter the United States, represents the vanguard of an evolving threat. Abdulmutallab’s ability to penetrate U.S. defenses and nearly pull off a devastating attack exposed serious failures in the U.S. intelligence community’s ability to share and make adequate use of clues about such plots.

Among them were a warning that Abdulmutallab’s father delivered to CIA officials in Nigeria about his son’s growing radicalism, as well as the interception of Al Qaeda communications in Yemen indicating that a Nigerian was being employed in a terrorist plot.

The White House released a report on the intelligence breakdowns last week. John Brennan, President Obama’s top counter-terrorism advisor, acknowledged that until the alleged attempted Christmas Day attack, U.S. analysts did not see Al Qaeda’s franchise in Yemen as capable of carrying out an overseas plot.

The Senate report makes clear that Al Qaeda in the Arabian Peninsula has recovered from being on the verge of collapse several years ago to represent a potent new threat.

The network in Yemen is led by Nasir Wahayshi, a former aide to Al Qaeda leader Osama bin Laden and one of 23 Al Qaeda fighters who escaped from a Yemeni prison in 2006.

The deputy of the group is Said Shihri, a Saudi citizen released from the U.S.-run prison at Guantanamo Bay, Cuba, in November 2007.

greg.miller@latimes.com