(Most recent update: 4:20 p.m. ET)
Investigators are still waiting to interview Boston Marathon bombing suspect Dzhokhar Tsarnaev, whose wounds reportedly include injuries to his neck and leg. An official tells CNN that Tsarnaev has been "intubated and sedated," rendering him unable to speak with them.
Massachusetts Gov. Deval Patrick described Tsarnaev's health Saturday as "serious but stable ... I think, not able to communicate yet."
Authorities are developing ideas about the possible motives of the 19-year-old Tsarnaev and his older brother, Tamarlan Tsarnaev, the suspect who died after a gunfight with police. Officials also want to know whether the two men acted alone when they allegedly planted bombs near the marathon's finish line.
Another question being asked is whether the attacks possibly could 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 he 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 Associated Press and other news outlets were reporting the request came from 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 the elder Tsarnaev brother.
"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 mementos 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 4:20 p.m. ET. Boston PD Chief Says Suspects May Have Planned More Violence:
The Tsarnaev brothers had enough weaponry to inflict more damage, Boston Police Commissioner Ed Davis says of the marathon bombing suspects.
"We have reason to believe, based upon the evidence that was found at that scene... the explosive ordnance that was unexploded, and the firepower that they had, that they were going to attack other individuals," Davis said on CBS's Face the Nation Sunday. "That's my belief at this point."
"They had IEDs, they had homemade hand-grenades that they were throwing at the officers," Davis said. "The scene was littered with unexploded improvised explosive devices that actually, we had to point out to the arriving officers and clear the area. They were strewn about the area. There was also one found in the motor vehicle that was abandoned, the Mercedes SUV."
Update at 2:20 p.m. ET. Memorial Draws Hundreds:
Hundreds of residents gathered Sunday to pray, to sing and to pay tribute to the three people who died Monday Martin Richard, 8; Lingzi Lu, 23; Krystle Campbell, 29 and MIT police Officer Sean Collier, 26, who was slain Thursday, as well as the more than 170 people who were injured.
Update at 12:20 p.m. ET. Bomb Suspects Acted Alone, Mayor Says:
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 underway Sunday, in a show of solidarity with the victims in Monday's Boston Marathon blasts. Many runners wore black ribbons in their honor.