(CBS News) The FBI is leading a worldwide investigation into the Boston Marathon bombings, checking all intelligence leads and possible informants, trying to find out who planted the bombs in Boston. They've been questioning one man who was said to be acting suspiciously at the scene, but no one has yet been identified as a suspect.
Boston officials said Monday's twin bombing attack came with no warning. Boston Commissioner Ed Davis said on Monday, "We talk about the threat picture all the time as we lead up to this particular event and we have no information that this was going to happen."
As federal and local investigators try to determine who was behind the attack, the lack of any credible threat leading up to the attack may be an important clue.
Former Homeland Security Secretary Tom Ridge says the absence of intelligence might suggest the attacker is not affiliated with a larger terrorist group: "It may lead to the fact that this was not connected to a major jihadist organization. This might very well may have been a domestic terrorist or lone wolf, as you might want to describe it."
A lone wolf like Eric Rudolph responsible for bombings in the mid-90s, including the explosion in Atlanta's Centennial Olympic Park.
Officials will also be looking at when Monday's attack occurred. This week on the calendar has been a hot-button for domestic extremists. The burning of the Branch Davidian compound in Waco, Texas, and the bombing of the federal building in Oklahoma City both took place the third week of April.
Those who analyze previous attacks say the lesson learned is to cast a wide net for potential culprits. CBS News national security analyst Juan Zarate said, "I don't think we can assume we know who perpetrated these events before we do the investigation. What we've learned is that various groups of all stripes -- domestic and international -- learn to adapt, but they have a common goal in mind, which is to kill a lot of people and get lots of attention."
Ridge says the reality is it's impossible to guarantee security at every public event.
"They probably had undercover police, they probably had observers on the roof. I suspect they had cameras, they had people roaming the area, but again -- open public venue, 500,000 people, you can't create a fail safe environment."
Investigators have swept up a large amount of potential evidence, including small bomb fragments and surveillance pictures and tape, but it's still too early to know if the attack was the work of a terror group -- domestic or foreign -- or if it was the act of a lone wolf who somehow may have been inspired to act out.
Watch Bob Orr's full report above.