Police in Austin have received hundreds of calls about suspicious packages as they seek to solve a series of bombings that have killed two people.

Packages made of cardboard have exploded at three homes in the Texas city this month, putting residents on edge. The first, on 2 March, killed 39-year-old Anthony House. Draylen Mason, 17, died when a box was opened inside his home on Monday morning. His mother was injured. About five hours later, a 75-year-old woman was seriously hurt as she picked up a package outside her house.

“From everything I’ve heard about Draylen he was an outstanding young man who was going places with his life and it’s an absolute tragedy that he is no longer with us,” Brian Manley, the Austin interim police chief, told reporters on Tuesday.

Police are asking residents to call 911 if they see a suspicious package, such as an unexpected delivery left on their doorstep overnight that was not delivered by a service such as the USPS, FedEx or UPS. As of Wednesday morning they said they had taken 370 calls, none resulting in the discovery of another device.

The attacks happened miles apart from each other in neighborhoods in the east and north-east of the city. The two who died were black and the 75-year-old woman is Hispanic. Authorities have not ruled out a racial motive.

“That is something we have to pay attention to. That does not indicate that it’s a hate crime. But we’re not going to rule that out because we don’t want to limit anything that we’re considering,” Manley said in the media briefing.

Police initially believed the first bombing was an isolated act of retaliation for a drugs raid they had carried out nearby, but now think the attacks are linked and are searching for connections between the victims. The two who died reportedly knew each other.

The city is currently hosting thousands of visitors to the annual South by Southwest festival. Rewards from state and local authorities worth $65,000 have been offered for information leading to an arrest.