WASHINGTON—President Obama Friday night observed the end of a harrowing week of violence in Boston, lauding the coordination of law-enforcement officials that led to the capture of the second suspect while pledging to seek answers about why two young men bombed innocent civilians at the Boston Marathon.

``We’ve closed an important chapter in this tragedy,’’ Obama said in a press briefing at the White House after 10 p.m.

``Obviously tonight there are still many unanswered questions, among them why did men who grew up and studied here, as part of our country ... resort to such violence?’’ the president said. ``How did they plan and carry out these attacks, and did they receive any help?’’

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``Whatever they thought they could ultimately achieve ultimately failed.’’

Obama also said American mourns the loss of life in the Texas fertilizer plant explosion, saying those victims ``will not be forgotten.’’

``All in all, this has been a tough week,’’ he said.

House Speaker John Boehner issued an emailed statement: ``We are also proud of the people of Boston and Watertown for showing great resolve and assisting authorities throughout the ordeal. This has been a long day and a long week, but along the way we have gained many examples of courage and character.’’