How long does it take terrorist groups to claim responsibility?

One reason many are speculating that the Boston Marathon bombing was the work of a "home-grown" or "lone-wolf" terrorist is the absence of a foreign terrorist group claiming responsibility for yesterday’s tragedy. But how long does it typically take foreign terrorists to own up to plots on American soil?

If history is any guide, claims of responsibility are not immediate.

For instance, many may forget the precise sequence of events following the 2001 terror attacks on the World Trade Center. But Osama bin Laden didn’t officially take responsibility for the attack until late October 2001 — almost two months after the assault.

Then there’s the 2009 "underwear bomb" attempt by Umar Farouk Abdulmutallab. That attack occurred on Christmas Day — a Friday — but the message by al Qaeda in the Arabian Peninsula claiming responsibility didn’t surface until Monday, three days later. (It had been originally dated Saturday but wasn’t published on radical Islamic websites until Monday.)

And how about the Fort Hood shooting in 2009? It took four days for Anwar al-Awlaki to publicly praise his radicalized pupil, Maj. Nidal Malik Hasan, on his English-language web site for the tragic killing of 13 people in Texas.

Not to put too fine a point on it, but we’re only one day out from the bombing of the Boston Marathon. It’s perfectly plausible that a claim of responsibility could be forthcoming. This is not to suggest that yesterday’s twin bombings weren’t domestic in origin — that may very well be the case. But it’s worth acknowledging the potential time lag for claims of responsibility.

In fact, sometimes terrorist groups never claim responsibility. After the attempted Times Square bombing by Pakistani-American Faisal Shahzad in May 2010, Attorney General Eric Holder declared that the "Pakistani Taliban was behind the attack. We know that they helped facilitate it. We know that they probably helped finance it and that he was working at their direction." Though independent reports have confirmed this, the Pakistan Taliban denied knowledge of the foiled bomb plot. "This is a noble job and we pray that all the Muslim youths should follow Faisal Shahzad. But he is not part of our network," the group said. It just goes to show, in situations like this, patience is a virtue.