Two package bombs left outside homes in Austin, Texas, exploded on Monday, killing a teenager and injuring two women in attacks that police linked to a deadly blast earlier this month.
The latest bombings came as the Texas capital hosted its annual South by Southwest music, technology and film festival downtown. The victims in all three cases were African-American or Hispanic, and police said they were examining the attacks as possible hate crimes.
In each of the three attacks, a cardboard package was left at the front of a private residence and exploded after an unwitting victim picked it up or tried to open it, Austin Police Chief Brian Manley told reporters.
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The packages that exploded on Monday were not delivered by any mainstream commercial shipping services, such as Federal Express or United Parcel Service, Manley said.
"We are looking at these incidents as being related," Manley said, adding that federal investigators had joined the search for suspects and a possible motive.
"We cannot rule out that hate crime is at the core of this, but we are not saying that that is the cause," Manley told a news conference.
He warned residents to watch out for boxes left outside their homes and to report anything suspicious to police.
The first explosion, on March 2, killed a 39-year-old black man at a house in the city's Harris Ridge neighborhood, which is home to technology companies and about 12 miles (19 km) northeast of downtown. The blast was powerful enough to blew out walls at the home's entryway.
A 17-year-old African-American boy died on Monday when he found a package outside his house in east Austin and brought it into the kitchen of his home, where it exploded, Manley said.
A woman in her 40s was in the home and injured by the blast, although she was expected to survive, police said.
A second explosion, about 4 miles (6 km) away in southeastern Austin, injured a 75-year-old woman in the predominantly working-class Hispanic community of Montopolis, police said. A neighbor, Brandon Rendon, told Reuters the victim was in a wheelchair watering her grass before the blast occurred.
Monday's attacks unfolded as the city was thronged by visitors to the South by Southwest Festival. On Monday afternoon, London Mayor Sadiq Khan delivered a keynote address at the event, which draws hundreds of thousands of people to the city each year.
Manley said the bombings posed no threat to the festival, but he urged visitors to remain vigilant.