31.   After the missile tube was found, U.S. commanders tightened security around the base.

32.   Airport officials tightened security in Chicago, in Kansas City, Mo., and in Florida.

33.   Airports have tightened security, with Zurich requiring passengers on U.S. flights to take off their shoes for x-raying.

34.   But Browne insisted that he and other lab officials did make attempts to tighten security.

35.   But several superintendents in other urban school districts said they have tightened security considerably in recent years to keep strangers from walking into their schools.

36.   But since Abdel Rahman was sentenced, the American authorities have tightened security at most airports in the United States as a protective measure against possible revenge attacks.

37.   But that same year, a strategy for tightening airline security, proposed by a vice-presidential panel, was largely ignored.

38.   Capitol building security was again tightened.

39.   Canada, meanwhile, moved to tighten security amid reports of terrorist groups operating within its borders.

40.   Companies are tightening security.