Austin bombings work of serial bomber, police chief says

Sunday's explosion triggered by tripwire, 2 injured

AUSTIN, Texas - Four bombings that have killed two people and injured four others in Austin this month are believed to be the work of a serial bomber, Austin Police Chief Brian Manley said.

Manley said at a news conference Monday that Sunday night's explosion that injured two men marks a "significant change" from the first three because it was triggered by a tripwire that would have hit any random person walking by it. The first three attacks were carried out with package bombs left on people's doorsteps.

Manley said as investigators search for a pattern in the attacks, they will try to determine if there is a specific ideology motivating them.

Related: SAPD sending bomb team to Austin in wake of latest explosion

The attack Sunday happened in a southwestern Austin residential neighborhood that isn't close to the sites of the first three attacks. Manley told "Good Morning America" on Monday that the men were riding or pushing bicycles when the explosives detonated.  

Manley called on the person or people behind the bombings to reach out to police to let them know why they're setting off the explosives.

He said that it's too soon to say whether Sunday night's bombing that injured two men could have been a response to his call for those behind the bombings to reach out.

Manley said investigators see "similarities" between Sunday night's bomb and three others this month. 

Frederick Milanowski, the special agent in charge for the Bureau of Alcohol, Tobacco and Firearms, said the latest bomb is "more sophisticated" because it used a tripwire.

Related: Here's what you need to know about Austin explosions

The men injured in Sunday's bombing, ages 22 and 23, are hospitalized in stable condition with significant injuries, Manley said.

Both men are white, as opposed to the victims of the three prior bombings, who were black or Hispanic. Those attacks killed two people and injured two others.

Austin Mayor Steve Adler said growing anxieties are "legitimate and real" following the fourth bombing.

Adler told The Associated Press on Monday that residents shouldn't think twice about calling 911 if they see anything suspicious. 

The University of Texas' campus police force is warning students returning from spring break to be aware of the four recent bombings to hit Austin, saying, "We must look out for one another."

In a tweet Monday, UT Austin police said, "When you get on campus this morning ASK your friends if they've heard about the bombings. TELL them about the incidents."

It also urged students to report suspicious items to authorities. Classes were resuming Monday after a week off.

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