Counterterrorism officials are poring over information on possible threats from the past 30 days in the wake of the Boston Marathon attack, to see if there are any clues to what happened while authorities continue to investigate.
According to multiple U.S. officials, there was no heightened "chatter" on major jihadist websites in the initial hours after the Boston attack.
This is one metric intelligence agencies look at in the wake of a terror attack, though it doesn't mean that overseas involvement has been ruled out.
One former senior intelligence official told Fox News there was no elevated "chatter" in advance of the attempted Times Square bombing in May 2010 or the attempted Christmas Day bombing in 2009, either.
The agency currently poring over threat information is the National Counterterrorism Center, the group specifically set up after the Sept. 11, 2001, terror attack to track and assess threats against the United States.
An investigative source familiar with the case also told Fox News that the FBI is scanning cell phone tower records for the "moment of detonation." If that is identified, the corresponding cell phone number can be traced.
Though President Obama did not initially call the Boston attack "terror" during brief remarks from Washington Monday evening, he used the term in a statement from the White House on Tuesday.
"Any time bombs are used to target innocent civilians, it is an act of terror," Obama said.
Authorities say the blasts left at least three dead and more than 170 injured.
An intelligence bulletin sent Tuesday morning from the FBI and Department of Homeland Security also cautioned law enforcement that the bombing highlights the use of "improvised explosive devices (IEDs) to target large gatherings, including at special events, in order to inflict mass casualties."
It cited recent plots, including the attempted Times Square bombing. The bulletin, obtained by Fox News, sent out a list of potential warning signs indicating such a plot.
"Terrorists have demonstrated continued interest in attacking significant infrastructure, economic, and symbolic targets. We encourage state and local law enforcement, as well as security personnel, to consider the following protective measures," the bulletin said.
Fox News' Catherine Herridge, Jennifer Griffin and Mike Levine contributed to this report.