Austin 'Tripwire' Bomb Attack: What We Know About the Texas Explosion

Police in Austin, Texas, said they were investigating reports that an explosion which injured two people was activated by tripwire.

The incident is being linked to three parcel bomb attacks that since March have killed two people and injured two others.

GettyImages-931100754 The scene near Galindo Street in Austin, Texas, where a woman in her 70s was injured in an explosion, on March 12. Getty Images

A timeline of attacks

The first package bomb exploded on March 2 at 6:55 a.m., killing Anthony Stephan House, 39, at his home on the 1100 block of Haverford Drive.

On March 12, Draylen Mason, 17, was killed, and a woman seriously injured, in a package-bomb explosion in the kitchen of a home on the 4800 block of Oldfort Hill Drive at 6:44 a.m.

At 11:50 a.m. the same day, Esperanza Herrera, 75, was severely injured in a package explosion while visiting her mother's home on the 6700 block of Galindo Street.

In the latest attack, two men in their 20s were seriously injured by the explosion on the 4800 block of Dawn Song Drive at about 8:30 p.m. on Sunday. Police said the device was left on the side of the road, and the two men were either cycling or pushing their bikes when the explosion ocurred. 

Both patients are in serious but not life-threatening condition, a spokesperson at St. David's South Austin Medical Center told CNN.

The investigation so far

The latest explosion came hours after police offered a $100,000 reward for information on the person or persons behind the attacks, with a $15,000 reward already offered by the state governor.

Two of the men killed so far were African American, and police have not ruled out racism as a motive.

In a press conference, Austin police chief Brian Manley warned of a different kind of attack.

 "It is very possible this device was activated by someone either handling, kicking or coming into contact with a tripwire,” he said.

"That changes things in that our safety message has been about the handling of packages. We need the community to have an extra level of vigilance and pay attention to any suspicious device. Do not approach it. Call 911."

Manley continued, "We do believe that this was a bomb that exploded, but it was not similar to previous explosions. We are working under the belief they are connected, but we have not processed the scene."

Earlier Sunday, before the latest explosion, Manley issued a rare direct appeal for the person or persons responsible to come forward and "reach out to us before anyone else is injured or anyone else is killed out of this event."

Hundreds of federal agents are assisting local police in the investigation, with officers investigating 735 reports of suspicious items since last Monday.

Investigators said the devices were left on doorsteps overnight and not delivered by traditional methods.

The Austin Police Department has issued information on how to spot suspicious packages and what to do if one is received.

Did the victims know each other?

Nelson Linder, local president for the National Association for the Advancement of Colored People, told NBC News that the two murdered men had links to prominent African-American families and attended the same Methodist church.

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