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The Senate observed a moment of silence for the victims just before an evening vote, and Speaker John A. Boehner, Ohio Republican, led a moment of silence in the House later Monday.

“It’s just a sad, horrific occasion that Boston’s experiencing today,” Sen. William M. Cowan, Massachusetts Democrat, said from the Capitol.

In Washington, crowds near the White House were pushed back to Lafayette Park with the Secret Service shutting down Pennsylvania Avenue during the peak of the spring tourism season.

In an abundance of caution, the Metro transit system beefed up its police presence through at least the Tuesday evening rush hour, said agency spokesman Dan Stessel.

“We’ll have additional police presence on the system until we have better assessment of what has happened in Boston,” he said, adding that there were no reports of specific or credible threats against the system.

⦁ Shaun Waterman, Susan Crabtree and Stephen Dinan contributed to this report.