But then, Dzhokhar Tsarnaev came barreling at them in the stolen vehicle, the chief said. The officers scrambled out of the way, and the vehicle then ran over the older brother and dragged him for a short distance.
Tamerlan Tsarnaev also had explosives on his body, officials have said.
Clues about radicalization?
While investigators piece together the brothers' actions leading up to the marathon bombings, details have emerged suggesting the elder Tsarnaev was turning radical.
The Tsarnaev family hails from the Russian republic of Chechnya and fled the brutal wars there in the 1990s. The two brothers were born in Kyrgyzstan, authorities said.
An FBI official said agents interviewed Tamerlan Tsarnaev in 2011 at the request of the Russian government. The FBI said Russia claimed that he was a follower of radical Islam and that he had changed drastically since 2010.
But the Russian government's request was vague, a U.S. official and a law enforcement source said Sunday. The lack of specifics limited how much the FBI was able to investigate Tamerlan, the law enforcement official said.
Tamerlan Tsarnaev apparently became increasingly radical in the past three or four years, according to an analysis of his social media accounts and the recollections of family members. But so far, there has been no evidence of active association with international jihadist groups.
In August 2012, soon after returning from a visit to Russia, the elder Tsarnaev brother created a YouTube channel with links to a number of videos. Two videos under a category labeled "Terrorists" were deleted. It's not clear when or by whom.
In January, Tamerlan Tsarnaev disrupted a service at the Islamic Society of Boston's mosque in Cambridge, Massachusets, a board member told CNN's Brian Todd.
Tsarnaev was reacting to a speaker who likened the Muslim Prophet Mohammed to Martin Luther King Jr., the board member said. He calmed after worshippers spoke with him, and returned often for pre-dawn prayers on Fridays, the board member said.
Dzhokhar Tsarnaev also sometimes attended prayers -- but only with his brother, the board member said.
Memorials and tributes
Boston officials planned a moment of silence for 2:50 p.m. Monday to mark the passing of one week since the bombings. A minute later, bells will toll to honor the victims.
One of those victims, Krystle Campbell was memorialized Monday morning in a service at St. Joseph Catholic Church in Medford, Massachusetts.
After the service, police officers lined the street in front of the church as other officers wearing dress uniforms saluted as the casket bearing her remains was taken from the church and loaded into a hearse.
Another memorial service was scheduled Monday night for victim Lingzi Lu, a student from China.
Also on Monday, runners in at least 80 cities will participate in the "Run for Boston in Your City" campaign, organizer Brian Kelley said. The global campaign is "a run for those that were unable to finish, a run for those that may never run again" and "a run for us to try and make sense of the tragedy that has forever changed something we love," according to organizers.
A week after the marathon bombings, 50 people remain hospitalized, including two in critical condition, according to a CNN tally.