There have so far been no claims of responsibility for the deadly Boston bombings, and investigators are considering a wide range of suspects, the FBI said on Tuesday.
"The range of suspects remains wide open," FBI special agent Rick DesLauriers told a press conference in Boston, saying the probe into Monday's blasts was proceeding "methodically, carefully, yet with a sense of urgency".
DesLauriers said investigators from the Federal Bureau of Investigation, as well as Boston and state police, had recovered items from both blast sites including black nylon bags in which the bombs are believed to have been hidden.
"Fragments of BBs and nails, possibly contained in a pressure-cooker device," were also recovered and sent to the FBI laboratory at Quantico, Virginia for analysis, DesLauriers told reporters.
More than 2,000 tips have been received as of noon Tuesday, he said, adding: "We will continue to work around the clock tirelessly, side by side with our partners, to continue to investigate and act on these leads."
"Regarding who might be suspected of this event, the investigation is in its infancy. (...) But rest assured we're working hard to get the answers," said the agent, who is in charge of the FBI's Boston office.