A 24-year-old Afghan man at the center of an unfolding FBI investigation into a possible U.S. terrorism cell was ordered held without bond in Colorado Monday as authorities raced to learn more about an alleged plot using hydrogen peroxide explosives and who else might have been helping to carry it out.

Meanwhile, authorities in Washington and elsewhere were stepping up safety patrols on mass transit systems in response to an advisory issued in connection with the probe.

This sounds like one of the more serious domestic terrorism plots that has been uncovered.