(CNN) The clues were in the receipts.
As police in Austin, Texas, desperately searched for the person who had left explosive packages all over the city, killing two people, they realized the bombs had something in common: They were being made from common household ingredients. So investigators hit the area's stores, scanning receipts and looking for clues.
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"Agents fanned out throughout the city of Austin going to big-box retail stores as well as locally owned stores trying to determine whether or not there were suspicious purchases," Tony Plohetski, an investigative reporter for the Austin American-Statesman, told CNN's "New Day."
A high-ranking law enforcement official told his newspaper that investigators were "going through receipts and going through sales records from those stores."
That worked, the law enforcement official said. Plohetski said the search of the receipts provided investigators with "critical evidence."
They had a person of interest, now identified as Mark Anthony Conditt, 23.
Federal search warrants were then used to get the man's IP address, which showed that he'd been making "suspicious" Google searches, Plohetski said.
Police also developed a sketch from witness interviews.
But the big break came when Conditt went to a mail delivery store south of Austin and shipped an explosive device -- and the whole thing was captured on security video.
"Police say that they used that as the final piece to put all of this together, really in the past 24 hours," Plohetski said.
Convinced this was their man, police worked quickly. Late Tuesday night they used cellphone technology to track Conditt to a hotel in Round Rock, about 20 miles north of Austin, according to the American-Statesman. There, they spotted the vehicle witnesses had told investigators he was driving.
Police and federal agents surrounded the hotel but didn't move in immediately. Because they were dealing with a suspected serial bomber, they wanted backup.
"(We were) awaiting the arrival of our tactical teams because we wanted to have ballistic vehicles here so we could attempt to take this suspect into custody as safely as possible," Austin Police Chief Brian Manley said in a news conference early Wednesday.
They were still awaiting the arrival of those teams when, some time later, Conditt took off in his vehicle. Police followed him as he drove on the service road along I-35, whizzing by the various restaurants, gas stations and hotels. Then the vehicle came to a stop in a ditch. A SWAT team cautiously approached.
Boom! Conditt detonated a device inside his vehicle. The blast from the explosion knocked down and injured one officer, while another fired a shot at the suspect.
Conditt, who suffered "significant injuries," died in the blast.
"This is the culmination of three very long weeks in our community," Manley said.
While officials have urged residents to remain vigilant, they said later Wednesday that Conditt made a 25-minute video confession in which he described making seven bombs, including the one that killed him.
Manley said authorities believe they have accounted for every bomb Conditt made.