BLACKSBURG, Va. (AP) - Representing America's anguish, President Bush told Virginia Tech students and teachers at a somber convocation Tuesday that the nation is praying for them and "there's a power in these prayers."

"Laura and I have come to Blacksburg today with hearts full of sorrow," he said in six-minute remarks at a convocation on the campus where 33 people, including the suspected gunman, died in two separate shootings the day before. "This is a day of mourning for the Virginia Tech community and it is a day of sadness for our entire nation."

Before flying to the tragedy-stricken university in southwestern Virginia, Bush also ordered flags flown at half staff and issued a written proclamation in honor of those killed and wounded.

Speaking to a somber basketball arena, packed with students and others, many wearing orange short-sleeved Virginia Tech T-shirts, the president quoted a recent student blogging about the killings to encourage those who grieve to reach out for help.

"To all of you who are OK, I'm happy for that," Bush said, quoting the Internet posting. "For those of you who are in pain or who have lost someone close to you, I'm sure you can call on any one of us and have help anytime you need it."

He urged those angered by the killings not to be overcome by evil.

"People who have never met you are praying for you," Bush said. "They're praying for your friends who have fallen and who are injured. There's a power in these prayers, a real power. In times like this, we can find comfort in the grace and guidance of a loving God."

Before the service, Bush received a briefing on the shootings and their investigation from Virginia Tech President Charles Steger.

Bush spoke on a day of raw emotion. He spoke to students who he said had just lived through the worst day of their lives.

"On this terrible day of mourning, it's hard to imagine a time will come when life at Virginia Tech will return to normal, but such a day will come," Bush said. "And when it does, you will always remember the friends and teachers who were lost yesterday, and the time you shared with them, and the lives that they hoped to lead."

Meanwhile, the federal Bureau of Alcohol, Tobacco, Firearms and Explosives has sent 12 agents to Virginia Tech and the FBI has contributed some 15 agents as well for the investigation. The federal help, including input from the U.S. Attorney's office in the Western District of Virginia, is being coordinated at a command center set up on the campus.

In addition to helping with the crime scene, the Department of Justice is making counselors available to victims and their families through a special office and the Education Department is offering assistance as well.

White House spokeswoman Dana Perino deflected any questions about Bush's view of needed changes to gun control policy, saying the time for that discussion is not now.

"We understand that there's going to be and there has been an ongoing national discussion, conversation and debate about gun control policy. Of course we are going to be participants in that conversation," she said. "Today, however, is a day that is time to focus on the families, the school, the community."

Perino added: "Everyone's been shaken to the core by this event and so I think what we need to do is focus on support of the victims and their families and then also allow the facts of the case to unfold before we talk any more about policies."

In times of tragedy, Americans turn to the president to be the nation's consoler and comforter.

Bush rallied the nation after the Sept. 11, 2001, terrorist attacks. One of the most enduring images of his presidency is Bush standing atop a pile of rubble in New York with a bullhorn in his hand. After Hurricane Katrina struck the Gulf Coast, Bush made repeated trips to the region but wound up criticized for the government's sluggish response to the storm.

President Clinton went to Oklahoma City in 1995 after the bombing of the federal building there, and his on-the-scene empathy was later viewed as the key factor in reviving his presidency and helping him win re-election.

Bush first spoke about the shootings on Monday afternoon, expressing shock and sadness about the killings from the White House. He lamented that schools should be places of "safety, sanctuary and learning" - similar to remarks he has made in the past after school shootings.