Amid fears in the aftershock of the airplane crash in New York, the United Nations engaged a partial lock-down of its headquarters Monday morning.

The U.N. Web site said U.S. law enforcement stopped all pedestrians and vehicles from entering the complex at 10 a.m., but later partially lifted the restrictions. Those inside the building were permitted to leave as needed.

Meanwhile, according to the Web site, U.N. staff was encouraged to continue work. Meetings at the center went uninterrupted.

Robert Lowry, associate professor of political science, said the partial lock-down was a simple precautionary measure. “Given the state of things these days, as soon as the plane went down they said, `We don’t know what caused the plane to crash and we don’t know whether it is another terrorist incident or not, so we just aren’t going to take any chances,’ ” Lowry said.

Although the United Nations has not voted to support or reject the way the “war on terrorism” is being waged, Richard Mansbach, professor of political science, said its headquarters would be a possible target.

“It was assumed the United Nations would be a logical target of terrorism because the organization – the building, in fact – was the site of a global meeting of heads of state, and the main topic was terrorism,” he said. “It’s where, a few days ago, President Bush delivered a speech in which he told all member-states that they had to support the United States or be regarded as against us.”

Mansbach said the restrictions could have been initiated because of thoughts the plane might have been the first of two or more. He said U.N. officials were concerned about the possibility that the first plane missed and another was planned to strike later.

He said that when the government is uncertain, it must prepare for a worst-case scenario.

“It had a lock-down of institutions in D.C. after the attacks on the Pentagon,” he said. “Its a perfectly reasonable precaution.”