To the very latest on the boston bombing investigation. And the headlines this morning that the tsarnaev brothers were originally planning to attack on the fourth of july, but moved their plans and... See More
To the very latest on the boston bombing investigation. And the headlines this morning that the tsarnaev brothers were originally planning to attack on the fourth of july, but moved their plans and their bombings up to patriots day. Abc's brian ross is tracking it closely. Brian, they actually achieved building those bombs faster than they thought they would. According to a person briefed from the case the surviving brother told the fbi the marathon became the target only because it didn't take them as long as they thought to build the bombs. They were eager to attack. ♪ Reporter: The original target was supposed to be a july 4th event in boston. According to the person briefed on the younger brother's statement. The famed boston pops concert and fireworks on the esplanade was not specifically named but it is one of the most prominent july 4th events in the country broadcast nationally by cbs with an estimated crowd of about 500,000 people. The point was to attack america, officials say, in keeping with the internet preachings of the now dead al qaeda recruiter anwar al awlaki who the brothers said was the inspiration to carry out jihad against the united states. We have chosen the path of war in order to defend ourselves. Reporter: The brother told the fbi that the bombs were constructed in the cambridge apartment of his brother tamerlan and that they were able to build the bombs from pressure cookers and fireworks quicker than they expected. They might not have had the resources as we've seen in other bomb attacks, but they knew how to make the bomb go boom. Reporter: According to the person briefed on the case, the younger brother also said they were eager to use the bombs before july and on april 12th drove around the area and considered attacks on the boston and the cambridge police headquarters and only then the brother reportedly told the fbi did they realize preparations were being made for the marathon and settled on the race as their target. If it is true as the brother reportedly said to the fbi that the bombs were built in tamerlan tsarnaev's apartment, it raises further questions widow -- what his widow knows. She's going by her maiden name and denies knowing about the bomb plot but the fbi continues to press her including questions she made in a phone call after the fbi started to look for him.
This transcript has been automatically generated and may not be 100% accurate.