Two package bombs that left a teenager dead and two others injured in Austin, Texas are believed to be linked to a case earlier this month, police say.
A 17-year-old boy died after opening a parcel and a woman in her 40s was wounded in the explosion, Austin Police Chief Brian Manley said.
A 75-year-old Hispanic woman was also hurt when a package exploded elsewhere in the city on Monday, he said.
Police did not rule out the possibility of all three attacks being hate crimes.
The two explosions on Monday came days after a similar blast killed a 39-year-old man at his home in the Texas capital.
“Based on evidence that we have… these incidents are related,” Chief Manley said at a news conference.
Investigators said at the time they believed that the explosion in north-east Austin on 2 March was an isolated incident.
But on Monday Austin police said they suspect the earlier fatality, initially treated as a suspicious death, was linked to Monday’s death. Both are now being investigated as homicides.
Investigators said they could not rule out the possibility of the blasts being hate crimes, as the victims in two of the cases were African American while the third blast injured a Hispanic woman.
The latest blast happened at 06:44 local time (11:44 GMT) on Monday.
The Austin police bomb squad, the Austin Fire Department, the FBI, the Bureau of Alcohol, Tobacco, Firearms and Explosives and postal inspectors responded to the scene.
Mr Manley told reporters: “Early this morning one of the residents went out front and there was a package on the front door step.
“They brought that package inside the residence and as they opened that package both victims were in the kitchen and the package exploded causing the injuries that resulted in the young man’s death and the injuries to the adult female.”
The house sustained significant damage, according to Chief Manley.
He told a news conference they do not know what the motive may be.
“We do know that both of these homes that were the recipient of these packages belong to African Americans,” Chief Manley added.
“So we cannot rule out that hate crime is at the core of this, but we’re not saying that that’s the cause as well.”
He said investigators had determined the latest package had not come through the US Postal Service, meaning it had probably been hand delivered.
The police chief advised the public to be wary of un-franked and unexpected packages.