Transcript for Spanish authorities say Barcelona attack could have been even worse

Meantime, we're learning the attack was supposed to be much longer. An accidental house explosion revealing a terror cell, their whereabouts. And authorities believe they decided they had to act fast. Here's Brian Ross. Reporter: Spanish authorities tonight now say it could have been even worse. That a much more devastating attack was being planned in this villa outside Barcelona by members of the isis-inspired terror cell. But their plans changed Wednesday night when one of the group's bombs detonated prematurely, killing one of the terrorists and causing the others to flee. When police responded. Left behind, at least 21 propane gas canisters that can be seen in the rubble. Canisters officials believe were intended for some powerful truck bombs. If these attackers would have been better bomb makers we would be looking at hundreds of deaths. Reporter: After the explosion at the rented villa in the town of alcanar, police say some members of the cell headed to Barcelona for the van attack against pedestrians. Other left alcanar and later ended up at the beach resort of cambrils where five of them, including this young man, were shot dead by police. Revealing a complex terror cell not previously known or even suspected. Clearly in this circumstance we are not dealing with the lone wolf offender. Traditional counterterrorism techniques will not pick up on these types of attackers. Reporter: Even as ISIS suffers defeat on the battlefield, its efforts to spur on its followers in Europe and the U.S. Have had success. This ISIS fighter, identified as an American, goaded his followers with a message posted in may. Are you incapable of stabbing a kafir with knife? Throwing him off a building? Or running him over with a car. Reporter: The message from ISIS to followers has been, you can stay home and do great damage by attacking your own country.

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