transit system have been briefed on the overseas threat. in what s being called an abundance of caution, they are stepping up a security search along amtrak and tightening security in new york city. we have done some tactical shifts and changes that we think are appropriate at this time. reporter: the travel alert for americans in europe is light on details. heavy on impact. i wouldn t say we have specific information about a particular place, a particular time. and yet we have i think sufficient information that justifies issuing of the alert. ruers the government s message, people should continue with plans to travel to europe but do so cashesly. be cautious and aware we are following the multiple streams of threat information. reporter: fbi and homeland security department official say there is no sign terrorists are targeting the u.s. or its citizens in the current plot. but the government is take no chances. yesterday, the u.s. attacked a suspected center of th