Police in southwest Austin responded Sunday night to the fourth explosion in less than two weeks, hours after authorities issued a direct appeal for whoever has been planting a series of bombs to stop. Two men in their 20's were hospitalized with serious, but non-life-threatening injuries according to Austin Travis County EMS. Authorities cleared a backpack in the area right after the incident.

Investigators including the FBI and the Bureau of Alcohol, Tobacco, Firearms and Explosives (BATF) responded to the scene, as over 500 federal agents have been dispatched to assist in the investigation.

At the scene of a reported explosion in southwest Travis County. We know two men were hurt. Police are keeping us streets away from the scene, which is on Dawn Song Drive. pic.twitter.com/55jN27XKrn — Pattrik Perez KVUE (@PattrikPerez) March 19, 2018

Austin Police Chief Brian Manley urged residents to remain in their homes "until we've had a chance to deem this neighborhood safe," which will be "at minimum until daylight."

Residents are warned not to touch or go near anything that looks like a package.

Initial Statement from Chief Manley regarding explosion https://t.co/CCydXhmQ8e — Austin Police Dept (@Austin_Police) March 19, 2018

Eliza May said she was watching a TV show in her home when she heard an explosion that sounded like a transformer blew up in her backyard. “It sounded like when the transformers go out, but it was five times magnified that,” said Ms. May, who lives about 200 feet from where the explosion was said to have occurred. Another neighbor, Lori Goodgame, said the explosion caused her house to shake. Her first thought was that lightning had hit her home. “There was a huge boom,” Ms. Goodgame said. -NYT

The explosion came hours after a Sunday news conference in which police publicly implored the bomber or bombers to contact authorities so they might learn more about the "message" behind the attacks.

“These events in Austin have garnered worldwide attention, and we assure you that we are listening,” Chief Manley said to the bomber or bombers. “We want to understand what brought you to this point, and we want to listen to you.”

So far, the bombings have killed two black men and wounded a 75-year-old Hispanic woman. While law enforcement hasn't concluded that the crimes are racially motivated, they haven't ruled it out.

Nelson E. Linder, the president of the Austin branch of the N.A.A.C.P., said on Sunday evening that he did not know the race of the two men injured in the latest explosion. “It’s important for the whole city to understand this is a danger, and I think tonight kind of confirms that,” Mr. Linder said. “I think that’s what this means tonight, that this whole city is at risk.” -NYT

A $100,000 reward has been issued for information leading to the arrest and conviction of whoever is responsible for the bombings, while the governor's office is offering $15,000 to bring the total to $115,000.

Prior to Sunday's incident, three previous bombs in Eastern and Northeastern Austin left two dead and a third seriously wounded; one on March 2 and two more occurring on March 12. The victims opened unsuspecting packages that were left on their doorsteps which were equipped with sophisticated homemade devices.

None of the packages were mailed, rather, they were reportedly placed right next to the victims' doors. In two of the bombings, the package exploded right after the victims picked them up, while the third went off after it had been carried inside and opened.

“Every bomber that makes these leaves a signature,” said ATF special agent Fred Milanowski, who is in charge of the Houston division. “Obviously, once they find something successful for them, they don’t want to deviate from that because they don’t want something to blow up on them.”

Milanowski added that a certain degree of skill was required to have assembled, moved and deposited the devices without an accidental explosion - while declining to reveal exactly what materials were used to make them.

“It wouldn’t be a typical household that would have all these components, but I would say that all the components are commercially available,” he said.

Since the March 12 double-bombings, Austin police officers have responded to 735 calls from anxious residents reporting hundreds of suspicious packages. Officials are urging residents to call 911 if they come across a package they were not expecting, or which was not delivered by a major services such as the USPS, FedEx or UPS.

“The scope goes beyond just Austin,” a law enforcement official who spoke on condition of anonymity told the New York Times. “We’re looking for anyone that could have been involved in making bombs in the past in Texas, and really anywhere in the United States.”