What was supposed to be a jam-packed week of controversial legislation on Capitol Hill was punctuated by moments of silence for the victims of the Boston Marathon bombings and trepidation as security was tightened in the hours after the attack at key locales in Washington.

“Thoughts & prayers are with everyone in Boston,” tweeted Sen. Chuck Schumer (D-N.Y.), a member of the Group of Eight. “Our bipartisan work on immigration will wait at least a day out of respect for the victims.”

Whip counting continued at a quiet yet frenzied pace for the background check compromise of Sens. Pat Toomey (R-Pa.) and Joe Manchin (D-W.Va.), expected to come up for a vote this week as the first amendment of the gun control bill, but indications were that they lacked support and may have to go back to the drawing board.

Both chambers paused to remember the victims even as details remained hazy in the early evening hours. “I hesitate to call what the event was but clearly it was a terrible tragedy,” Boston Rep. Mike Capuano (D-Mass.) said on the House floor, adding “whether official terrorism or unofficial” the attack was committed by “an evil person.” House Speaker John Boehner (R-Ohio) ordered flags over the Capitol to be flown at half-staff.

The worst attack on U.S. soil since 9/11 was poised to dramatically shift the predefined agenda of the 113th Congress after a campaign season where the president said al-Qaeda was on the run and national security ranked low for voters concerned with economic recovery.

“I’ve updated leaders of Congress in both parties, and we reaffirmed that on days like this there are no Republicans or Democrats — we are Americans, united in concern for our fellow citizens,” President Obama said in an early evening statement in the briefing room.

“We still do not know who did this or why. And people shouldn’t jump to conclusions before we have all the facts. But make no mistake — we will get to the bottom of this,” he continued. “And we will find out who did this; we’ll find out why they did this. Any responsible individuals, any responsible groups will feel the full weight of justice.”

The White House released a photo of Obama on the phone in the Oval Office with FBI Director Robert Mueller. The FBI asserted federal jurisdiction to swiftly take over the investigation, with the bureau calling it a criminal investigation and “potential” terrorism investigation.

There was no alert issued from the National Terrorist Advisory System, which was the administration’s replacement for the Bush-era color-coded terror alert system — but there’s never been an NTAS alert since the system began nearly two years ago.

“Our thoughts and prayers go out to everyone impacted by this incident in Boston, especially the families and loved ones of those injured,” Homeland Security Secretary Janet Napolitano said in a statement. “Federal, state, and local law enforcement agencies continue to respond, and at the President’s direction, the Department of Homeland Security is providing any support necessary in this ongoing investigation. We encourage the public to be vigilant, and to listen to direction from state and local officials.”

As of Monday evening, three people were confirmed dead, including an 8-year-old boy, and more than 140 were injured, including 8 children, in a pair of sequential bombings at the finish line of the Boston Marathon. Reports indicated ball bearings were packed in the explosive devices.

The April 15 date of the attack touched off rampant speculation about motive and timing: It was Patriots’ Day in Boston, as well as tax day nationwide. It was Israel’s 65th Independence Day. It was Kim Il-Sung’s 101st birthday. It was the day the U.S. bombed Libya in 1986.

But target-wise, it was an internationally recognized event with inherent controlled chaos where Bostonians off work and school for the civic holiday were gathered for an annual tradition.