Here's how police tracked down the Austin bombing suspect before he blew himself up

Store receipts helped police track down the 24-year-old suspect they believe is responsible for five explosions that killed two people and injured several others over the course of nearly three weeks.

What's the story?

Police discovered the bombs were all made from common household ingredients, CNN reported. That's when investigators started searching receipts at local stores.

"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, investigative reporter for the Austin American-Statesman, told CNN Wednesday.

Plohetski said a high-ranking law enforcement official told the Statesman that investigators were "going through receipts and going through sales records from those stores."

The investigators turned up "critical evidence" in the searches, which led to a person of interest, according to CNN.

Federal search warrants were used to search the suspect's IP address, which showed the man had made "suspicious" Google searches, Plohetski said.

The police interviewed witnesses who helped them develop a sketch of the suspect.

The suspect's "fatal mistake was when he walked into a FedEx office to mail the package" containing an explosive device, Rep. Michael McCaul, (R-Texas) told Fox News.

Surveillance video caught the whole thing, including the suspect's license plate.

"Police say that they used that as the final piece to put all of this together, really in the past 24 hours," Plohetski said.

Investigators used cellphone technology to track the suspect to a motel about 20 miles north of Austin, late Tuesday night, where they spotted his car.

What happened when police found the suspect's car?

While police waited for a tactical unit to arrive and move in, the suspect fled the hotel in his vehicle. Police followed the suspect until he ran his car into a ditch alongside Interstate 35.

The SWAT team started to approach, and that’s when he detonated the device inside his car and blew himself up, according to police.

One SWAT officer suffered minor injuries when he was knocked back by the blast. Another officer fired at the suspect.

What else?

The suspect's motive remains unclear, according to police.

But they are actively working to determine a motive and whether the suspect worked alone or with others.

"Investigators have detained two roommates of the Austin bombing suspect. One roommate was detained, questioned and released. The other is currently being questioned. Their names will not be released because they are not under arrest at this time. APD PIO," the Austin Police Department tweeted.

Police are urging residents to remain vigilant because there may be other devices that haven't been located.