AUSTIN, Texas — Two of the three families targeted by package bombs left on Austin doorsteps knew each other and were connected through local activism in the black community, a civic leader said Tuesday, but it was not clear how they might be tied to a third household where a package bomb also exploded.

Investigators have said the three explosions that killed two people and wounded two others could have been hate crimes since all the victims were black or Hispanic. But they also said they have not ruled out any possible motive.

Dixon Mason, a prominent dentist in east Austin, was grandfather of 19-year-old Draylen Mason, who was killed Monday after carrying a package left at his home into the kitchen and opening it. The elder Mason was friends with Fredie Dixon, stepfather of 39-year-old Anthony House, who died in a similar attack in another part of the city on March 6, said Nelson Linder, president of the Austin chapter of the NAACP.