Another bomb went off in the city of Austin, Texas Sunday night, the fourth one this month. Police are already saying the latest bomb has similarities to previous ones. It also showed an additional level of sophistication because this one was activated by a tripwire. From the Washington Post:

The explosion in an Austin neighborhood Sunday night had “similarities” with the three bombs that detonated in the Texas capital earlier this month, leading authorities to determine that they are dealing with a “serial bomber” terrorizing the city, police said Monday.

The latest blast, which injured two men who were walking along the road in a residential area, plunged the city further into a frightening mystery that has forced residents to remain locked in their homes as investigators scoured the area for answers.

The explosion on Sunday night was apparently set off by a tripwire on the road, leading police to believe the bomber or bombers had “a higher level of sophistication, a higher level of skill” than authorities initially believed, said Brian Manley, the interim Austin police chief. He also said this explosion marked an apparent shift in tactics after the three previous devices were left at people’s homes.

A reward is being offered for information on the bomber. Texas Gov. Greg Abbott tweeted this out last night:

The three previous bombs were all designed to look like packages, though none of them were sent through any package delivery service. Since the bomber was leaving the packages at specific locations, that led authorities to believe he might be targeting specific individuals.

When I wrote about this story last week, there was some concern the attacks might be race related. The first two people killed in this bombing spree, Anthony House and Draylen Mason, were both African-American. The third victim was an elderly Hispanic woman. At that point, authorities said they weren’t ruling out racial bias as a factor but they weren’t claiming that was the motive either. Today’s victims, two men in their 20s, were both white. From CNN:

“We said from the beginning, we’re not willing to rule anything out. Because when you do, you limit your focus,” Manley said. “This changes the concerns that we had initially. Though we have still not yet ruled it out.”

So the racial angle on these attacks looks less likely now. Obviously, using a tripwire suggests the bomber isn’t targeting anyone in particular or even any race, he’s just looking to injure people at random. So, for now, police aren’t calling this a hate crime or terrorism. They are not ruling any motive in or out.

Austin Police Chief Brian Manley was asked about possible suspects during a press conference this morning. He says his office has received a lot of tips since last week and is following up on every one. So far they have had persons of interest but haven’t identified a clear suspect. Here’s the press conference from this morning laying out what authorities know so far.