A deadly serial bomber has struck in Austin for what appears to be the fourth time this month, frightening residents and prompting police to put a neighbourhood in the Texan capital into lockdown.

After three packages exploded earlier in March, killing two people and seriously injuring another while baffling the authorities, the city’s police chief appealed on Sunday for the suspect or suspects to make contact.

But later that night another bomb seriously injured two more people, deepening the mystery – and the alarm.

The men, aged 22 and 23, were by the side of a road in a quiet residential area, known as Travis Country, about seven miles west of downtown just after 8.30pm on Sunday when, police said, they triggered a tripwire next to a fence. They were taken to hospital with significant injuries and were in stable condition on Monday morning.

The location and the detonation method are in marked contrast to the three previous incidents. Anthony House, 39, died on 2 March. Then 17-year-old Draylen Mason was killed on the morning of 12 March and his mother was hurt. About five hours later, Esperanza Herrera, 75, was seriously injured. All three picked up packages left on their doorsteps that exploded; the package that killed the teenager detonated in his kitchen.

Police had begun exploring possible connections between the victims. The two who died were African American and their families know each other, raising suspicions of a racial motive.

The attacks took place in the east of a city in which the Interstate 35 freeway that runs through downtown has historically acted as a dividing line between predominantly white areas to the west and mainly black and Hispanic neighbourhoods on the other side.

Sunday’s victims, though, are white; the neighbourhood is to the west, 10 miles from the previous attack; and the use of a tripwire in a public street suggests a perpetrator aiming for random targets.

“We are clearly dealing with what we expect to be a serial bomber at this point,” Brian Manley, the police chief, told reporters on Monday.

The latest explosion raises the fear that simply walking outside – particularly in the dark when a thin tripwire will be almost impossible to spot – could prove fatal. Police have appealed for residents to provide footage from home surveillance, such as doorbell cameras. Rewards for information worth $115,000 have been offered.

Austin welcomed thousands of visitors last week for the annual South by Southwest festival. A man was arrested on Saturday for an emailed bomb threat that prompted the cancellation of a concert by the Roots, but police said the suspect was not linked to the explosions.

The Travis Country neighborhood was cordoned off by police and residents were told to stay indoors.

“We were not willing to classify this as terrorism, as hate, because we just don’t know enough. And what we have seen now is a significant change from what appeared to be three very targeted attacks to what was last night a target that would have hit a random victim that happened to walk by,” Manley said.

However, the NAACP, the civil rights organization, on Monday called the bombings acts of domestic terrorism.

Police were guarding the main entrance to the subdivision in Travis Country on Monday afternoon, with FBI trucks parked behind yellow crime scene tape. One local, who gave his name as Joe said: “This morning when I left the house I was even afraid to open the door, thinking there might be a wire there.”

“We heard it at home. From a block and a half away it sounded very loud. it sounded like it was two [bangs] as the sound travelled.”

The incident took place in an affluent area where the city’s dense urban core gently gives way to gated communities where secluded mansions nestle in rolling hills and abundant greenery features hiking and biking trails.

“This is a family neighbourhood,” Joe said. “I can guarantee you it’s affecting the whole community … I have a 13-year-old and I have to worry about him, how it’s going to affect him.”



