Austin’s police chief said Monday morning that they believe they’re dealing with a “serial bomber” after the Texas city was rattled Sunday night by the fourth blast in less than three weeks.
The latest explosion, which left two men with serious but non-life-threatening injuries, used what’s known as a trip-wire device, which is different in sophistication from the prior three bombs that were left on porches and detonated when opened. Despite that, Austin Police Chief Brian Manley said there were enough similarities between the bombs to lead investigators to believe they were made by the same person or group.
“We are clearly dealing with what we see as a serial bomber,” Manley said during a press conference Monday morning.
Authorities also see the more sophisticated trip-wire device as a clear escalation in tactics from the cruder package bombs delivered over the past few weeks.
“A trip wire doesn’t necessarily suggest a military background,” said Manley. “But it suggests that the suspect or suspects we are dealing with have a higher level of sophistication than we believed, as they’re changing their methods to a more difficult device.”
Before Sunday, authorities had been urging caution around handling package deliveries. Now they’re telling residents to be careful about where they walk, as a trip-wire device can be activated by simply stepping on a wire connected to the bomb. Earlier on Monday, Austin Mayor Steve Adler told AP that the citywide anxiety was “legitimate and real.”
“We’ve seen a significant change from what appeared to be three very targeted attacks to an attack that would have hit any random victim who walked by,” Manley said.
Unlike in the earlier explosions, the victims in the latest bombing were both white.
Sunday night’s bombing came just hours after Chief Manley made a televised appeal to the perpetrator (or perpetrators) of the earlier bombings, which killed two people and injured one.
“We hope this person or persons is watching and will reach out to us before anyone else is injured or anyone else is killed,” Manley said. “We assure you, we are listening and we want to understand what brought you to this point, and we want to listen to you, so please call us.”
This article originally appeared on VICE News US.