WASHINGTON — Riders on the Washington metro system were warned Thursday to expect random checks of carry-on items two days after the government disclosed the arrest of an Afghan man for allegedly threatening to bomb the system.

The stepped-up security measures at the height of the Christmas shopping season also coincided with a warning by Interpol that it had received information about possible Al-Qaeda attacks in the United States and Europe.

Metro Transit Police "will conduct random inspections of carry-on items, as part of the continuously changing law enforcement programs designed to keep the system safe," the Washington Metropolitan Area Transit Authority said in a statement.

Police will use bomb-sniffing dogs and sensors that rely on ionization technology to check the bags for hazardous materials, it said.

The inspections would take only minutes and were designed to be "non-intrusive," it said.

"Carry on items will generally not be opened and physically inspected unless the equipment indicates a need for further inspection," it said.

People can refuse the inspections, but they will be prohibited from bringing their bags into stations, the agency said.

On Tuesday, the Justice Department disclosed that it arrested Awais Younis, an Afghan man, after he described on Facebook how to build a pipe bomb that could maximize casualties on the Washington Metrorail.

During a chat last month Younis said the third and fifth cars on the Metro had the highest number of commuters on them and that he could place bombs there without being noticed, according to the affidavit.

"Complainant responded by saying 'You wouldn't do that,' and (Younis) responded by saying 'Watch me,'" the affidavit says, without providing further details on who turned Younis in.

He had also recently posted a message saying: "Christmas trees were going to go boom."