Commissioner Ed Davis told CBS' "Face the Nation" that authorities found an arsenal of homemade explosives after a gun battle between police and the suspects in the Boston suburb of Watertown early Friday.
"We have reason to believe, based upon the evidence that was found at that scene - the explosions, the explosive ordnance that was unexploded and the firepower that they had - that they were going to attack other individuals," Davis said. "That's my belief at this point."
One suspect, 26-year-old Tamerlan Tsarnaev, was fatally wounded in the gun battle, and his brother, 19-year-old Dzhokhar, escaped, authorities said. The two threw explosive devices at police in their getaway attempt, authorities said.
"There were over 250 rounds of extended ammunition that was found at the scene. This was a five- to 10-minute gun battle that occurred there, punctuated by loud explosions," Davis said, adding that the explosive devices were homemade.
The scene was loaded with unexploded bombs, and authorities had to alert arriving officers to them and clear the scene, Davis said. One improvised explosive device was found in the Mercedes the brothers are accused of carjacking, he said.
"This was as dangerous as it gets in urban policing," Davis said.
He said on "Fox News Sunday" that authorities cannot be positive there aren't more explosives that haven't been found. But the people of Boston are safe, he said.
Dzhokhar Tsarnaev was captured Friday evening while hiding in a boat in Watertown after a massive manhunt. He is hospitalized in serious condition and has been unable to communicate with authorities.
Davis said shots were fired from the boat, but investigators haven't yet determined where the gunfire was aimed.
The federal Bureau of Alcohol, Tobacco, Firearms and Explosives is tracing the weapons to try to determine how they were obtained by the suspects.