Two people have been injured after Texas' capital city Austin was rocked by its fourth bombing this month.
The latest device, which used a tripwire, showed a "different level of skill" to the three previous ones, according to the city's police chief.
Brian Manley told ABC's Good Morning America that both men who were injured in Sunday night's explosion in the south-western Austin neighbourhood of Travis Country are white, unlike the victims in the three earlier attacks, who were black or Hispanic.
Residents were warned to remain indoors and to call police if they needed to leave their homes before 10am. After sunrise, officers scoured the area for anything suspicious.
Travis Country is far from the sites of the earlier bombings, which occurred over two weeks in residential areas east of the Interstate 35 road, which divides the city.
At a news conference hours after Sunday's blast, Mr Manley repeated his warning for people to not pick up or approach suspicious packages.
He said: "We want to put out the message that we've been putting out and that is, not only do not touch any packages or anything that looks like a package, do not even go near it at this time."
Mr Manley also said authorities had worked to "clear" a suspicious backpack found in the area that was part of a separate report.
Police kept residential streets on lockdown, gradually expanding their barricades and closing off all roads into the area. Before daybreak, Austin police sent another alert to mobile phones, advising residents to continue staying indoors and to call police if they needed to leave their homes before 10am.
Austin's school district announced that buses would not be going into the Travis Country area because of police activity and that any absences "due to this situation" will be excused.
The two men injured on Sunday are in their 20s and are white. Police said they were taken to hospital with non-life-threatening injuries.
The first of the four explosions to hit Austin this month was caused by a package bomb that detonated at a house in the city's north-east on March 2, killing a 39-year-old man. Two more package bombs then exploded farther south on March 12, killing a 17-year-old, wounding his mother and injuring a 75-year-old woman.
Police said all three of those explosions were likely related and involved packages that had not been mailed or delivered by private carrier but left overnight on doorsteps.
Mr Manley originally suggested they could have been hate crimes since all the victims of the first three explosions were black or Hispanic, but now says investigators are not ruling out any possible motive.
The latest explosion came hours after authorities raised the reward by 50,000 dollars for information leading to the arrest of whoever is responsible for the first three explosions. It now totals 115,000 dollars.