Austin Police Department responded to reports of an explosion Sunday night in a Travis Country neighborhood that injured two people, APD Chief Brian Manley said in a media briefing Monday morning.

This was the fourth incident of an explosion in Austin since March 2, when a package exploded at an East Austin home. Unlike the previous three, Sunday’s explosion was probably set off by a trip wire, Manley said.

“In the past we have talked about not touching suspicious packages, not moving packages, not handling packages,” Manley said. “The belief that we are now dealing with someone using trip wires shows a higher level of sophistication, a higher level of skill.”

Manley said investigators have seen similarities in the explosion Sunday night and the previous three explosions this month, all of which occurred in residential areas. He said the suspect behind the incidents may be a serial bomber.

APD is asking anyone in the Travis Country neighborhood to send in any home security footage to assist in finding a suspect for the bombings, Manley said.

Following the explosion Sunday night, the UT Police Department released a safety statement to the UT community. The statement was written by UTPD Chief David Carter and James Johnson, interim associate vice president for campus safety and security.

UTPD’s statement said the use of a trip wire does not require victims to handle packages to trigger an explosion.

“This enhances the warning from being limited to the safe handling of packages to now include not going near any packages, unattended bags, unattended backpacks or anything that looks out of place,” UTPD’s statement said.

Since the fourth explosion took place in southwest Austin, the warning area has expanded, UTPD said. The previous three incidents only took place in northeast and East Austin.

UTPD and APD have also urged people to call 911 to report any suspicious items or people.

Over 500 federal agents from the FBI and the Bureau of Alcohol, Tobacco, Firearms and Explosives are working on solving the string of explosions, Manley said. Additionally, assistance is being provided to APD by the San Antonio Police Department, the Houston Police Department and the Texas Department of Public Safety, Manley said.

“We need this to stop,” said Christopher Combs, FBI special agent, during the media briefing. “We are very concerned that people could get hurt by this just by walking now that we have trip wires.”