Authorities on Tuesday revealed little new information about the terrorist attack at the Boston Marathon, the most high-profile bombing on U.S. soil in more than a decade and a tragedy that presents a new test for President Obama.

Obama, like all Americans, is standing by, waiting for answers as to who planted two bombs improvised from pressure cookers at the finish line of the storied race, and why. Three Americans were killed in Monday's attack and more than 170 others were injured.

"Anytime bombs are used to target innocent civilians, it is an act of terror," Obama said, labeling the attackers as terrorists for the first time. "What we don't yet know, however, is who carried out this attack or why, whether it was planned and executed by a terrorist organization -- foreign or domestic -- or was the act of a malevolent individual."

Obama will travel to Boston on Thursday to speak at an interfaith service dedicated to victims of the bombing.

But even as Washington officials were sorting out what happened in Boston, lawmakers were told Tuesday that a letter laced with the poison ricin was sent to Sen. Roger Wicker, R-Miss., and authorities are now trying to track down the letter's source.

"It was caught in the screening facility," said Sen. Claire McCaskill, D-Mo., who was briefed by Homeland Security Secretary Janet Napolitano and FBI Director Robert Mueller. "That's why we have an off-site screening facility for mail."

Unlike a string of foiled terrorist plots in recent years -- and other high-profile acts of violence where the perpetrator was immediately known -- Americans now are left to wonder what motive the attackers had. By Tuesday, no one had publicly claimed credit for the bombing.

Clearly, though, the bombings present a new challenge for Obama, who despite more than four years as commander in chief in a post-9/11 world has never dealt with an attack on the mainland.

"For those that think the president is a superhero, this is a moment of reality," said Charles Walcott, a presidential expert at Virginia Tech, where another act of violence shocked the nation six years ago. "All he can do is act in a symbolic capacity, the healer-in-chief, and promise to catch whoever did this."

Though smaller in scale than the Sept. 11, 2001, terrorist attacks, the twin explosions in Boston revived for many Americans the anxiety and fear from a decade earlier about the possibility of follow-on attacks on the homeland.

Across the country, major cities were on high alert, taking trash cans off the streets, increasing patrols near prominent landmarks and on public transportation and asking the public to report any suspicious activity.

Analysts preached patience, saying counterterrorism policies begun by former President George W. Bush and continued by Obama would ultimately prevail.

"There's no such thing as 100 percent prevention," said Matt Levitt, a senior fellow and director of the Stein Program on Counterterrorism and Intelligence at the Washington Institute for Near East Policy. "There will be an immediate spike of concern. But I personally don't think it's going to take a long time [to find the person behind the attacks]."

Others weren't so confident.

"It will become a challenge for the White House the longer we go without any idea who perpetrated this crime," Walcott said.