US Congress grapples with 'difficult week'

The US Congress struggled Thursday to come to terms with a tragic, tense week that included the nation's first major terror attack since 9/11, intercepted poison packages and a deadly Texas explosion.

Democratic Senate Majority Leader Harry Reid and top Republican Mitch McConnell opened their remarks by offering prayers to victims of a devastating blast at a fertilizer factory the previous evening that rocked the small town of West, Texas near the city of Waco. As many as 15 people were killed.

Congressional leaders noted the extraordinary series of events, including the twin bombs in Boston that killed three people and wounded 180, which left many on Capitol Hill on high alert as they recalled the September 11 attacks and the anthrax poisonings, both from 2011.

"Given the horrendous event of the Boston Marathon on Monday, followed by the event near Waco last night it's been a very difficult week for all of us," a somber McConnell said on the Senate floor.

"Our hearts are a little bit heavier."

Senators were supposed to be reveling in the rollout of a landmark immigration reform bill this week, but instead the focus was on tragedy and ongoing terror threats.

Tucked between the Boston and Waco blasts was perhaps the most immediate threat to the US Capitol complex in a dozen years: an anonymous letter laced with the deadly poison ricin sent to Senator Roger Wicker, as well as one sent to President Barack Obama.

"Americans should understand that this incident does not appear in any way to be related to the tragedy in Boston," said Reid.

"Nevertheless, it is a reminder to the Senate community, and to all Americans, to remain vigilant during these unsettling times."

The envelopes were intercepted at off-site mail facilities, and the Department of Justice announced the arrest of a Mississippi man in the case.

But on Wednesday, after calls by the Senate Sergeant at Arms to be extra vigilant, two Senate office buildings were placed on lockdown when suspicious packages were detected.

Two senators also announced there were suspicious packages found at offices in their home states. Those and the ones in the Senate buildings all tested negative.

First-term Senator Elizabeth Warren of Massachusetts delivered her maiden floor speech Wednesday, and while her words were heavy with mourning, she described how the bloodshed in Boston, and the actions taken to help victims, "unites the world in our common humanity."

Jitters continued Thursday morning when discovery of a white, powdery substance prompted evacuation of a US naval mailroom near Washington. A hazardous materials team deemed the substance harmless.

Leaders in the House of Representatives acknowledged the trying period. "It's been a rough week for the country," said Speaker John Boehner.

"Our hearts go out to the victims and the people of Boston. I'm glad the president's up there today, and I add my prayers to his."

A visibly moved Nancy Pelosi echoed her counterpart.

"Life is fragile," she said. "You never know from one day to the next."

Adding to the emotional quotient on Capitol Hill, relatives of the 20 children and six adults slaughtered in the Newtown school shooting spent much of the week making tearful pleas to senators to pass landmark gun legislation that would expand background checks for gun buyers.

The measure failed.