U.S. investigators say the bomb that exploded late Sunday in Austin, Texas was set off by a tripwire and was "more sophisticated" than three previous blasts earlier this month.
Authorities say two men, both in their early 20s, were injured in the latest explosion to rock the capital of the southwestern U.S. state. Austin police chief Brian Manley said both victims sustained "significant injuries" in the incident, but were in stable condition in a local hospital.
He said investigators "have seen similarities in the device that exploded here last night and the other three devices that have exploded in Austin starting on March 2." The earlier explosions killed two people and injured two others.
FBI agent Christopher Combs said the use of a trip wire to detonate the bomb in the latest blast "changes things. It’s more sophisticated. It’s not targeted to individuals. We’re very concerned that with tripwires, a child could be walking down a sidewalk and hit something." The previous explosions were package bombs left on people's doorsteps and were set off when the victims picked them up.
Combs said it was "very important" that Austin residents stay away from anything they might "consider to be suspicious."
Police are also investigating the bombings as possible hate crimes. The first three explosions killed two African American men and left a 75-year-old Hispanic woman fighting for her life. But the latest victims are white.
Authorities have offered a $100,000 reward for anyone who can offer law enforcement agencies information leading to arrests and convictions of those responsible for the explosions.
"We need this to stop," Combs said.
Authorities have called out to the person and people detonating the bombs to reach out to police to talk about why they are setting off the explosions.