WASHINGTON - Security tightened Sunday on Capitol Hill as grief-stricken lawmakers, their spouses and staffers were briefed on their own safety after the shooting of Rep. Gabrielle Giffords.

About 800 members of Congress, aides and spouses discussed security in a conference call with Capitol Police Chief Philip Morse, FBI officials and other congressional security staff members.

During the call, House Speaker John Boehner, R-Ohio, said he had asked security officials to "conduct an in-depth security review" for members of Congress when they meet Wednesday. A briefing will also be held for lawmakers' district office directors.

At the White House, President Obama called on Americans to observe a moment of silence at 11 a.m. EST today (9 a.m. in Tucson). He called it "a time for us to come together as a nation in prayer or reflection, keeping the victims and their families closely at heart." The president plans to observe the moment with his staff on the White House South Lawn.

House Democratic Caucus Chairman John Larson, D-Conn., said during the conference call that new members of Congress and spouses were especially concerned about security.

Congressional leaders are accompanied by plainclothes Capitol Police officers. Rank-and-file members typically don't get protection outside the Capitol, and there was no indication that policy will change.