WASHINGTON >> President Barack Obama declared tonight that the capture of a second suspect in the Boston Marathon bombings “closed an important chapter in this tragedy.” But he acknowledged that many unanswered questions remain about the motivations of the two men accused of perpetrating the attacks that unnerved the nation.

“The families of those killed so senselessly deserve answers,” said Obama, who branded the suspects “terrorists.”

The president spoke from the White House briefing room after 10 p.m. on the East Coast, just over an hour after law enforcement officials apprehended 19-year-old Dzhokhar Tsarnaev.

The suspect had been holed up in in a boat in a Watertown, Mass., neighborhood. His older brother, 26-year-old Tamerlan Tsarnaev, was also wanted in the bombings and was killed earlier Friday in an attempt to escape police, authorities said.

The capture of the second suspect capped a frenzied week in Boston, Washington and elsewhere around the country.

Letters addressed to Obama and Sen. Roger Wicker, R-Miss., were found to contain traces of poisonous ricin in tests, evoking eerie parallels to the anthrax attacks that followed the terror attacks of Sept. 11, 2001. And a massive explosion leveled a Texas fertilizer plant, leaving at least 14 people dead and more than 200 injured.

In his remarks Friday, Obama told the people of West, Texas, that “they are not forgotten.”

But he focused the bulk of his remarks on the day’s events in Massachusetts, saying the nation owed a debt of gratitude to law enforcement officials and the city of Boston, which was paralyzed Friday by the manhunt.

The president’s praise for law enforcement was echoed by Republican leaders on Capitol Hill. House Speaker John Boehner of Ohio called their actions “a job well done under trying circumstances,” while Senate Minority Leader Mitch McConnell of Kentucky said the nation “marveled at the coordination, skill, and bravery of military, intelligence, and law enforcement officials at every level.”

Homeland Security Secretary Janet Napolitano commended national, state and local law enforcement officials and first responders “who have been and continue to work tirelessly to get to the bottom of the senseless attacks in Boston, and defend and protect the American public.”

FBI Director Robert Mueller also praised the agencies and officials involved in the investigation and cited public cooperation as important to the successful outcome.

“The investigation will continue as part of our efforts to seek answers and justice, and there will be no pause in that effort. But tonight, I wish to thank all those who worked so tirelessly throughout the week in the pursuit of safety and justice,” Mueller said.

Three people were killed and more than 180 injured in Monday’s twin marathon explosions. The manhunt to track down the suspects resulted in the death of an MIT policeman and severely wounded another officer.

The two suspects were identified by authorities and relatives as ethnic Chechens from southern Russia who had been in the U.S. for about a decade and were believed to be living in Cambridge, Mass.

Obama urged the public against rushing to judgment as officials seek answers to the many questions that remain.

“When a tragedy like this happens, with public safety at risk and the stakes so high, it’s important that we do this right,” he said. “That’s why we take care not to rush to judgment — not about the motivations of these individuals, certainly not about entire groups of people.”