Following the tragic deaths of three people and the injuries of 176 victims, law enforcement is still searching for answers on who orchestrated Monday's Boston Marathon bombing.

According to CNN, the horrific attack left the Boston police with "the most complex crime scene that we've dealt with in the history of our department," said Commissioner Ed Davis.

On Wednesday morning, a source of federal law enforcement stated that a lid to a pressure cooker thought to have been used in the tragic explosions was located on the roof of a building at the scene.

Although the crucial piece of evidence does help the investigation progress, it still remains uncertain on whether Monday's bombing was by someone inside the U.S. or foreign.

"All of the talking heads that discuss this incident and incidents like it, if your experience and your expertise is Middle East terrorism, it has the hallmarks of al Qaeda or a Middle East group. If your experience is domestic groups and bombings that have occurred here, it has the hallmarks of a domestic terrorist like Eric Rudolph in the 1996 Atlanta Summer Olympics bombings," former FBI Assistant Director Tom Fuentes told the site.

The U.S. government had reportedly given the federal agencies fair warning in the past that pressure cookers, which are airtight pots used to rapidly cook or preserve food, have been stuffed with explosives and shrapnel, ultimately detonating with blasting caps.

Pictures obtained by CNN reveal the remnants of a pressure cooker located at the scene as well as a shredded black backpack and what appears to be metal pellets.

The remains of at least one pressure cooker are being sent to the national laboratory of the FBI in Virginia, where technicians will attempt to reconstruct the deadly devices.

A law enforcement official believes that the Boston Marathon explosives were more than likely detonated by timers. However, details pertaining to the detonating system used are still unknown at this time.

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Boston Marathon Bombing