People gather at a makeshift memorial for victims of the Boston Marathon bombings at the edge of the still-closed section of Boylston Street. The surviving suspect in the case, Dzhokhar Tsarnaev, remains in the hospital.
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Massachusetts Gov. Deval Patrick described Tsarnaev's health Saturday as "serious but stable... I think, not able to communicate yet."
Officials are developing ideas about the possible motives of Tsarnaev and his older brother and fellow suspect Tamarlan Tsarnaev, who died after a gunfight with police. They also want to know if the two suspects acted alone when they allegedly planted bombs near the marathon's finish line.
Another question being asked is whether the attacks could possibly have been prevented.
As we've reported, the FBI acknowledged Friday that its agents interviewed Tamarlan Tsarnaev in 2011, after being asked by a foreign government to investigate him as a potential risk. The FBI says it was told that Tsarnaev was "a follower of radical Islam and a strong believer, and that he had changed drastically since 2010 as he prepared to leave the United States." The FBI's inquiry did not uncover any terrorist activities.
The FBI did not identify the foreign government in an official statement, but the AP and other news outlets have identified it as Russia, a country Tamarlan Tsarnaev visited several times. The Tsarnevs are ethnic Chechens, "with links to the volatile North Caucasus region of Russia," as NPR's Corey Flintoff reported Saturday.
A senior congressional aide tells The Boston Globe that members of Congress are asking law enforcement officials about the FBI's earlier investigation of Tsarnaev.
"The FBI had this guy on the radar and somehow he fell off," the aide says. "We heard for several days leading up to this there was no intelligence. Now we know there could have been intelligence."
In Boston, the site of the twin attacks is still being processed for evidence. Nearby, mourners and well-wishers have left a pile of flowers, notes and mementoes in the days since the attack.
An interfaith memorial service is planned for 12:30 p.m. ET Sunday at the intersection of Boylston and Berkeley streets, where the members of six churches are gathering to honor the attack's victims.
We'll update this post as news about the attack and the suspects develops today.
Update at 12:20 p.m. ET. Bomb Suspects Acted Alone, Mayor Says:
The suspects in the Boston Marathon bombing acted alone, Boston Mayor Tom Menino says on ABC's This Week.
Menino provided no details about the health of suspect Dzhokhar Tsarnaev other than to say, "we don't know if we'll ever be able to question the individual."
When reminded by ABC's George Stephanopoulos that Massachusetts does not have the death penalty as an option, Menino said, "I hope that the U.S. attorney, Carmen Ortiz, takes him on the federal side and throws the book at him."
Update at 9 a.m. ET. London Marathon Marked By High Security, Memories Of Boston:
The London Marathon observed 30 seconds of silence before the race got under way Sunday, in a show of solidarity with those injured in Monday's Boston Marathon. Many runners wore black ribbons to honor the three people killed and the more than 170 injured in two bombings in Boston.