A teenager was killed and an elderly woman was critically injured Monday when two package bombs went off in the Texas capital of Austin, police said.
The blasts are connected to a March 2 explosion that took the life of Anthony House, 39, Police Chief Brian Manley said. The man had received a delivery at his home in the northeast section of the city.
Monday's violence killed a 17-year-old boy and badly injured a 75-year-old woman in separate blasts.
"This is the third in what we believe to be related incidents," Manley said at a Monday press conference.
The latest victims have not been identified.
The teenager died at the scene and a woman in her 40s was taken to a nearby hospital following an early morning attack in east Austin. Police did not elaborate on the woman's injuries.
Shortly before noon, the elderly woman was injured after a retrieving a box that had been left on her porch, police said.
Manley said there is no known motive for the explosions, but he said local and federal agents are exploring whether the incidents could be racially motivated since the first two victims were African-American and the elderly woman is Hispanic.
He implored residents to be especially careful about handling any unexpected deliveries left in front of their residences, adding that the explosive devices as being housed in boxes of an average size. There is no indication, he said, that the packages had been delivered by the U.S. Postal Service or private carriers like UPS.
"It's not time to panic," Manley said. But it's time to be vigilant and it's time to pay attention. It's time to come together as a city and solve this."
The FBI and the ATF are also investigating.
Monday's explosions came as the hugely popular SXSW festival has gotten underway in the area. The annual event brings hundreds of thousands of visitors to the film, music and technology event.