When asked whether the explosions were a terror attack, Davis said: "We're not being definitive ... but you can reach your own conclusions."
Security has been increased at the Pilgrim Nuclear Power Station in Plymouth, Mass., the only nuclear power plant in the state.
According to the Associated Press, cellphone service had been shut down in the Boston area to prevent any potential remote detonations of explosives. However, officials with Verizon Wireless and Sprint Nextel said there had been no such requests.
While Davis initially said there was a third explosion at the JFK Library around 4:30 p.m. Eastern Time, Boston police now say it was a fire, and they're not sure whether it was related to the other blasts.