At border crossings in El Paso, Texas, people waited in line up to an hour and a half - twice the usual delay - as officials thoroughly inspected nearly every car at the bridges. Inspectors lifted hoods, opened trunks and looked under floor mats, as guards wearing bulletproof vests and carrying assault rifles looked on.

Jim Michie, a U.S. Customs spokesman in Washington, said traffic was being allowed to pass ''but under the highest alert possible.''

The situation was much the same at the Canadian border, although officials acknowledged difficulties in monitoring such an extensive boundary.

The Border Patrol office in Grand Forks, N.D., ordered agents to work overtime - but the office's jurisdiction includes some 917 miles of the border in North Dakota, Minnesota and Wisconsin.

''It concerns me greatly - that we have this large border - that we don't have the resources to completely ensure that it's monitored like I would like to see it,'' said Glen Schroeder, chief patrol agent in Grand Forks.

The Border Patrol uses planes, electronic monitoring devices and night vision equipment to monitor the international border between ports of entry.

Authorities temporarily closed the Detroit-Windsor Tunnel that links Michigan to Ontario, then reopened the two-lane tunnel beneath the Detroit River. Federal agents stopped and inspected each Canada-bound car and truck.

Great Lakes shipping traffic was slowed when the U.S. section of the St. Lawrence Seaway was closed, preventing international ships from entering or leaving U.S. waters, officials said.

And one El Paso crossing was closed briefly Tuesday morning as inspectors checked abandoned vehicles that officials were concerned could conceal bombs, said Letty Zamarripa, a spokeswoman for the Immigration and Naturalization Service. The two cement mixers and a car were found to be clean, she said.

Among the border crossers were people anxious to reunite with family.

''In school everybody was crying and praying,'' said Sahara Lee Sanchez, 14, who was crossing back to her home in Juarez after her El Paso high school closed.