Austin police search for bombing motive, say explosives made with 'skill and sophistication'

Authorities work on the scene of an explosion in Austin, Texas. Photo / AP

The third package to explode in Austin this month, severely injuring a 75-year-old woman, was addressed to a different home nearby, according to people familiar with the investigation - adding a wrinkle to investigators' efforts to identify who is responsible for leaving sophisticated bombs on the doorsteps of unsuspecting residents.

The FBI and police have not identified a suspect or motive behind the attacks, and the clues revealed publicly have only deepened the mystery.

The two people killed in the explosions, a 39-year-old construction worker and a 17-year-old high school student, had relatives who were good friends and prominent members of Austin's African-American community, leading family members to speculate that they were targeted, perhaps in part because of their race.

But a victim in the third attack, the 75-year-old Hispanic woman, had no apparent connections to the other two. The woman, Esperanza Herrera, was visiting her mother's house, and people familiar with the case said the package she picked up was addressed to a different home.

Investigators have been poring over the victims' backgrounds and the construction of the bombs, hoping to find a clue that might lead them to the person or people responsible. The FBI sent behavioural profilers from Quantico, Virginia, as well as bomb technicians and evidence teams, said Christopher Combs, special agent in charge of the bureau's San Antonio office.

Police warned Austin residents to be wary when approaching packages left at their doorsteps and said they had received 265 calls about suspicious packages. None were deemed to be dangerous or related to the investigation.

Austin Police Chief Brian Manley said that police were not ruling anything out in the case, including that the attacks could have been motivated by racial hatred or terrorism. He said that whoever is behind the attacks has been able to construct and deliver deadly bombs without setting them off at any point in that process, which shows some level of bombmaking prowess.