Other residents said they were avoiding opening packages or even answering their front doors out of fear. “Today I ran to the window to make sure it was my postman dropping off the package on the doorstep before I opened the door,” posted one Twitter user, Anton Queer Proseer. Another Twitter user, Lavente McCreary, posted the message, “Literally checking the shipping numbers of all my packages right now to make sure we know when they’re coming. Stay safe, Austin.”

For a city of 947,000 that prides itself on a hipster eccentricity and an irreverent live music scene that spawned the local motto, “Keep Austin Weird,” the attacks and their aftermath have been unsettling. The new, subtle fear in places far from the scene of the explosions was that nowhere was safe. The speculation that the first three attacks were hate crimes because two of the victims were black and one was Hispanic seemed to fade as word spread that the two victims on Sunday were white.

“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 federal agents are assisting in the investigation, in addition to bomb technicians from the Houston and San Antonio police departments. As of Sunday, investigators were pursuing more than 435 leads that had been called in and had interviewed more than 230 people. Even before the fourth bombing, residents have been on the lookout: Austin officers have responded to 735 calls of suspicious packages since March 12, the day of two back-to-back bombings.

Officials said that at about 8:30 p.m. on Sunday, two unidentified men, aged 22 and 23, appeared to have tripped a wire in the 4800 block of Dawn Song Drive in the Travis Country neighborhood. They were either on the sidewalk or on a grassy area between the street and a fence when a device attached to the wire detonated. Both men suffered significant injuries but were in stable condition, the authorities said.

The Austin police chief, Brian Manley, said the use of the tripwire suggested “a higher level of sophistication than maybe we initially thought, based on them changing their methods to a more difficult device.”