Secret Service officials anxiously oversaw what was by far the largest security operation for a presidential inauguration, marshaling the forces of several dozen federal, state and local law-enforcement and intelligence agencies. Their biggest white-knuckle moment: when Obama and the first lady left a heavily fortified limousine and walked six blocks on the way to the...

WASHINGTON — The swearing-in of Barack Obama came off without a security-related hitch Tuesday. Underneath the calm veneer, however, federal authorities were intensively investigating a report that Somalia-based militants wanted to launch some kind of inauguration-related attack.

Secret Service officials anxiously oversaw what was by far the largest security operation for a presidential inauguration, marshaling the forces of several dozen federal, state and local law-enforcement and intelligence agencies.

Their biggest white-knuckle moment: when Obama and the first lady left a heavily fortified limousine and walked six blocks on the way to the White House.

As they strolled, government countersnipers patrolled rooftops and hyper-alert Secret Service fanned out, scouring parade viewers often more than a dozen deep.

The day’s responsibilities for most security officers ended at 4:39 p.m. when the Obamas walked under the white awning and entered the White House. For others, it will continue, as authorities search for any indication that the Somalia-based threat was real.

Russ Knocke, chief spokesman for the Department of Homeland Security, said authorities were taking the threat seriously, but that it was “of limited specificity and uncertain credibility.”

One federal law-enforcement official said the threat involved individuals affiliated with al-Shabaab, an Islamist extremist group active in Somalia. Authorities have become concerned in recent years that U.S.-based Somalians are traveling to Somalia to fight with extremists against U.S.-ally Ethiopia.

The official said the threat did not specify Washington, D.C., and could apply to numerous inauguration-related festivities, and that there was no indication that anyone connected with the threat has tried to enter the United States.

The Washington Post contributed to this report.