An explosion on Sunday night injured two people in a neighborhood in southwest Austin, just hours after law enforcement officials made an unusual direct demand to whoever was responsible for several deadly package explosions this month that have kept the capital of Texas on edge.
The authorities said they responded to Dawn Song Drive after reports of an explosion around 8:30 p.m. A short time later, two men in their 20s were taken to a hospital with serious injuries, the Austin-Travis County Emergency Medical Service said.
The Austin police chief, Brian Manley, said at an evening news conference that the police were asking residents within a half-mile radius of the explosion to stay indoors until officers could conduct a search for bombs.
Eliza May said she was watching a TV display in her home when she heard what sounded like a transformer blowing up in her backyard. “It sounded like when the transformers go out, but it was five times magnified that,” said Ms. May, who lives about 200 feet from where the explosion was said to have occurred.
Another neighbor, Lori Goodgame, said the explosion caused her house to shake. Her first thought was that lightning had hit her home. “There was a big boom,” Ms. Goodgame said.
Moments later, dozens of police cars, ambulances and fire trucks swarmed her street, Ms. May said. Police officers ordered neighbors — who had come outside to see what happened — to return to their homes, she said.
Investigators from the F.B.I. and the Bureau of Alcohol, Tobacco, Firearms and Explosives also responded to the neighborhood known as Travis Country, which is about five miles southwest of downtown Austin and in a different area from the three previous explosions. Bomb technicians with the A.T.F. were conducting a secondary sweep, officials said.
Because the device exploded after nightfall, the chief said, the police could not fully inspect the scene and would have to wait until sunrise on Monday to better examine it. It was not immediately clear if Sunday’s explosion was directly connected to the three previous bombings.
“We have not had an opportunity to really look at this blast site to descrive what has happened,” Chief Manley said at the evening news conference. “It’s obvious that there’s been an explosion, and it’s obvious it’s caused significant injuries to two people.”
At a news conference before Sunday’s explosion, the Austin police made a rare public appeal for the bomber or bombers responsible for the first three explosions to contact the police so officials could learn more about the “message” behind the attacks.
“These events in Austin have garnered worldwide attention, and we assure you that we are listening,” Chief Manley said in addressing the unknown bomber or bombers at the earlier news conference. “We want to understand what brought you to this point, and we want to listen to you.”
Chief Manley told reporters that he hoped the person or people responsible were watching, and that they would get in touch by calling 911 or reaching out online. He said investigators had not established a motive for the explosive packages.
“There’s the message behind what’s happening in our community, and we’re not going to understand that until the suspect or suspects reaches out to us to talk to us about what that message was,” Chief Manley said. “We still do not know what ideology may be behind this and what the motive was behind this.”
Before Sunday, three separate bombings this month in the eastern and northeastern parts of the city left two people dead and a third seriously wounded. In each case, the victims handled packages that were left on their doorsteps and were outfitted with homemade but sophisticated explosive devices.
Officials said the first bombing, on Haverford Drive on March 2, and two more on March 12, on Oldfort Hill Drive and Galindo Street, were connected. None of the packages were mailed. Instead, they were apparently placed directly near the doors of homes for the victims to find. In two cases, the bombs detonated when the victims picked them up; in the third, the package exploded after it had been carried inside and opened.
More than 500 federal agents are assisting the investigation from agencies including the F.B.I. and the A.T.F. Fred Milanowski, the A.T.F. special agent in charge of its Houston division, said he trusted that the same person built all three devices.
“Every bomber that makes these leaves a signature,” Mr. Milanowski said. “Obviously, once they find something successful for them, they don’t want to deviate from that because they don’t want something to blow up on them.”
Mr. Milanowski said a degree of skill was required to assemble, transport and deliver the devices without an accidental explosion. He declined to identify the materials that were utilized to make them.
“It wouldn’t be a typical household that would have all these components, but I would say that all the components are commercially available,” he said.
Since March 12, the day when two bombings occurred, anxious residents have reported hundreds of suspicious packages to the authorities; Austin police officers have responded to 735 such calls. Officials have continued to urge residents to call 911 if they receive a package that they were not expecting and that did not come to have been delivered by the Postal Service or a legitimate commercial service like U.P.S. or FedEx.
Law enforcement officials said they were looking for probable links to similar residential package bombings across the country.
“The scope goes beyond just Austin,” said a law enforcement official who spoke on condition of anonymity to discuss a continuing investigation. “We’re looking for anyone that could have been involved in making bombs in the past in Texas, and really anywhere in the United States.”
Asked at the news conference whether investigators were looking for links to bombings elsewhere, Chief Manley said they were pursuing all avenues. “We are not going to rule anything out until we have a reason to rule it out,” he said, “because when we do that, it narrows our focus and we may limit considering things that we should be considering.”
Over the past 30 years or so, package bombings have killed or wounded more than two dozen people across the country, excluding those connected to the Unabomber case. Many of the attacks have been solved by the authorities; they often stemmed from domestic disputes, and sometimes involved pipe bombs in packages wrapped like holiday presents.
The bombings in Austin have alarmed black leaders because the two people killed were African-American and the seriously wounded victim was a 75-year-old Hispanic woman. Law enforcement officials said that they did not have conclusive evidence that race played a role in the bombings, but that they were continuing to explore the possibility.
Nelson E. Linder, the president of the Austin branch of the N.A.A.C.P., said on Sunday evening that he did not know the race of the two men injured in the latest explosion. “It’s important for the whole city to understand this is a danger, and I think tonight kind of confirms that,” Mr. Linder said. “I think that’s what this means tonight, that this whole city is at risk.”
Investigators are examining connections between the two black victims, who both belonged to prominent African-American families. Officials said investigators were also looking into the possibility that the bomb that wounded the Hispanic woman may have been intended for someone else, but that nothing definitive had been established.
Chief Manley said on Sunday that the combined rewards offered for information leading to an arrest and conviction in the case had been increased to $115,000, from $65,000.
A concert featuring the Roots that was part of the South by Southwest festival in Austin was canceled on Saturday after the concert venue received a bomb threat in an email, the authorities said. No device was found, and the police later arrested a man on a charge of making a terroristic threat. The police said the man, Trevor Weldon Ingram, 26, was not a suspect in the package bombings.