Two package bombs have exploded kilometres apart in the capital city of Texas, killing a teenager and injuring two women in attacks Austin police linked to a deadly blast earlier this month.
In all three cases, a package was left at the front of a residence and exploded after an unsuspecting victim picked it up or tried to open it, Austin Police Chief Brian Manley told reporters.
"We are looking at these incidents as being related," Manley said, adding that federal investigators were among those looking for suspects and a possible motive for the attacks.
All the victims, including a 39-year-old man who died in the March 2 blast, were either African-American or Hispanic, Manley said.
"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 reporters.
He warned residents to watch out for unexpected boxes left outside their homes, without picking them up, and to report anything suspicious to police.
The attacks took place as Austin hosted thousands of out of town visitors for its annual South by Southwest festival.
In Monday's suspected attacks, a package bomb killed a 17-year-old youth and injured a woman at their Austin home. Later, an explosion put a 75-year-old Hispanic woman in critical condition with "life threatening injuries", Manley said.
"This is the third in what we believe to be related incidents that have happened over the past 10 days," he said.
Monday's blasts were in homes about 6km apart in east Austin, while the March 2 incident occurred at a house in the city's northeast Harris Ridge neighbourhood, about 19km from downtown.
The March 2 blast was initially investigated as a suspicious death but is now being treated as a homicide, Manley said.