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.

Officials with the Department of Homeland Security and the FBI sent a bulletin to transit agencies Friday repeating past warnings to be on guard for attacks on mass transit systems, and identifying hydrogen peroxide-based explosives as a specific risk. Federal officials called the notice “precautionary”, and said it included possible countermeasures such as random checks of stations, trains and buses.

Local officials in Washington said the bulletin specifically mentioned Grand Central Station in New York City, but said they have nevertheless increased the number of random patrols. Because there was no specific threat for this region, Metro spokeswoman Lisa Farbstein said police have not implemented random bag searches.

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