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By Doug Criss, CNN

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(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 that 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.

Austin bomber kills himself with explosive device

“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.” They were “going through receipts and going through sales records from those stores.”

That worked. Plohetski said the search of the receipts provided investigators with “critical evidence.”

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They now had a person of interest. 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 of the suspect from witness interviews.

Seen on videotape

But the big break came Tuesday, when the suspect, 24-year-old Mark Anthony Conditt, went to a FedEx store in south Austin and shipped an explosive device — and the whole thing was captured on security video.

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“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 that this was their man, police worked quickly. Late Tuesday night they used cell phone technology to track him 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 didn’t move in immediately. Since 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, the man 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.

Then, the suspect 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.

The suspect, who suffered “significant injuries,” died in the blast. His identity has not yet been released.

“This is the culmination of three very long weeks in our community,” Manley said. But he stressed the city’s nightmare may not be over.

He urged residents to remain vigilant, saying they don’t know where the bomber has been for the past 24 hours and if he may have sent additional packages.

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