‘Serial Bomber’ Is Suspected in Explosions That Have Put Austin on Edge

A growing sense of alarm spread across Texas’ capital city Monday as the authorities confirmed that a new makeshift bomb that injured two people in southwest Austin on Sunday appeared connected to three earlier explosions, but suggested a “higher level of sophistication” with the use of a tripwire triggering device.

“We are clearly dealing with what we expect to be a serial bomber at this point,” Brian Manley, Austin’s interim police chief, told reporters as parts of Travis Country, where the latest bombing occurred, were placed on lockdown through midafternoon.

Residents there had been asked to remain indoors through the early part of the day and look out for suspicious packages, like backpacks or boxes — meaning nearly every parcel in this city of 947,000 was suddenly an object of potential suspicion.

The latest warnings signaled a growing level of concern among the authorities, who were scrambling to identify similarities in tactics and targeting in the four attacks, which have now left two people dead and four others wounded. The three original attacks, which began March 2, featured simple package bombs triggered when victims interacted with the devices; the two men injured in Sunday evening’s attack appear to have triggered a tripwire, the police said.

“This is shocking and horrific and I share the same anxiety and anxiousness that the whole community feels,” Mayor Steve Adler said. “We want this stopped.”

More than 500 officers and agents have been called in to assist with the investigation, including F.B.I. profiling experts from Quantico, Va., who were trying to build a profile of a possible perpetrator, said Christopher Combs, special agent in charge of the bureau’s San Antonio office.

“We don’t know why the bomber is doing this. We don’t know his reasons,” said Mr. Combs, adding that the authorities were especially hoping to understand why whoever was behind the bombings was wielding “this level of violence.”

“We would really like the bomber to contact us so we can talk to him,” he said.

So far, the police have been alerted to more than 600 packages deemed suspicious, as residents have phoned and emailed friends and family to verify the provenance of parcels before opening them.

Chief Manley signaled that the authorities could be getting closer to labeling the attacks as domestic terrorism. “We will have to determine if we see a specific ideology behind this, or something that will lead us along with our federal partners to make that decision,” he said.