FILE - In this file photo of Feb. 26, 1993, a New York City police officer leads a woman to safety following a bomb blast at the World Trade Center. Twenty years ago next week, a group of terrorists blew up explosives in an underground parking garage under one of the towers, killing six people and ushering in an era of terrorism on American soil. (AP Photo/Alex Brandon, File)

FILE - In this file photo of Feb. 27, 1993, police and firefighters inspect the bomb creater inside an underground parking garage of New York's World Trade Center the day after an explosion tore through it. Twenty years ago next week, a group of terrorists blew up explosives under one of the towers, killing six people and ushering in an era of terrorism on American soil.(AP Photo/Richard Drew, File)

FILE. In this file photo of Feb. 27, 1993, a crater is exposed in an underground parking garage of New York's World Trade Center the day after an explosion tore through it. Twenty years ago next week, a group of terrorists blew up explosives under one of the towers, killing six people and ushering in an era of terrorism on American soil.(AP Photo/Richard Drew, File)

FILE. In this file photo of Feb. 26, 1993, the twin towers of the World Trade Center in New York City are shown in the aftermath of an explosion earlier that day. Twenty years ago next week, a group of terrorists blew up explosives in an underground parking garage under one of the towers at the World Trade Center, killing six people and ushering in an era of terrorism on American soil. (AP Photo/Ron Frehm, File)

FILE - In this file photo of Feb. 26, 1993, Victims of a fire at the World Trade Center in New York are treated at the scene after an explosion rocked the complex. Twenty years ago next week, a group of terrorists blew up explosives in an underground parking garage under one of the towers, killing six people and ushering in an era of terrorism on American soil. (AP Photo/Marty Lederhandler, File)

FILE - In this file photo of Feb. 26, 1993, firefighters in a cherry picker remove an explosion victim on a gurney outside one of the World Trade Center's twin towers in New York, after an explosion rocked the complex. Twenty years ago next week, a group of terrorists blew up explosives in an underground parking garage under one of the towers, killing six people and ushering in an era of terrorism on American soil. (AP Photo/Alex Brandon, File)

FILE - In this file photo of Feb. 26, 1993, two New York City police officers help an injured women away from the scene of the World Trade Center explosion. Twenty years ago next week, a group of terrorists blew up explosives in an underground parking garage under one of the towers, killing six people and ushering in an era of terrorism on American soil. (AP Photo/Joe Tabacca, File)

In this file photo of Feb. 26, 1993, helicopters jockey for position over the World Trade Center in New York following a noontime blast, which rocked the twin towers complex. Twenty years ago next week, a group of terrorists blew up explosives in an underground parking garage under one of the towers, killing six people and ushering in an era of terrorism on American soil. (AP Photo/Ron Frehm, File)