A fourth package bomb placed in Austin, Texas in the last three weeks went off Sunday, injuring two people in their 20s, according to interim Austin Police Chief Brian Manley. The incident adds another attack to a disturbing string of package bomb attacks that have brought terror to the city.

The Latest Bomb Was Seemingly Set Off Using A Trip Wire

Unlike previous incidents, Sunday's bomb was reportedly left on the side of the road, rather than on a doorstep, and was seemingly set off after two young bikers triggered an explosion my touching a trip wire of some sort, according to The Washington Post.

They sustained serious, but not life-threatening, injuries.

Police say they believe Sunday's bombing is connected to the three other bombings that have recently rocked Austin.

A Total Of Four Packages Have Exploded Across The City, Leaving Two Dead

Last Monday, 75-year-old Esperanza Herrera was seriously injured after picking up a package that was left outside her home, but addressed for somewhere else. Police Chief Brian Manley said "she was significantly injured[.]"

Hours earlier, a similar package killed 17-year-old Draylen Mason and seriously injured another woman.

On March 2nd, a 39-year-old man, Anthony Stephan House, was killed in a similar incident.

Neighbors told Dallas News reporter Jackie Wang that "the earth shook" and people "were running and screaming, holding babies and pets.

Mason And House Have Family Connections

The Washington Post reports that Anthony Stephan House and Draylen Mason have relatives who are friends, raising the possibility that the bombings were targeted.

House's stepfather, Freddie Dixon, told The Post last week that he was close to Mason's grandfather, Norman Mason. They were fraternity brothers, and Norman Mason also attended the church where Dixon was once a pastor, Dixon said.

[The Washington Post]

All Previous Victims Were People Of Color

Police Chief Manley told reporters that two of the previous victims were African American and that the third victim was Hispanic, leaving open the possibility that the incidents were racially motivated. The races of the latest victims have not been disclosed.

At Least Two Of The Packages Are Believed To Have Been Hand Delivered

Manley says that at least two of the packages are believed to have been delivered by hand, without the use of any major courier or mailing service.

Authorities Say They Suspect The Incidents Are Connected

Manley told reporters that evidence points to a connection between the incidents, and Texas Attorney General Ken Paxton has urged Austin residents to beware of unexpected packages and call 911 if they suspect something suspicious.

Authorities are offering up to $15,000 for information that leads to an arrest. Anyone with information has been encouraged to call Crime Stoppers at 1-800-252-8477, text "DPS" and a tip to 274637 or submit a tip online.

The FBI and the Burau of Alcohol, Tobacco, Firearms and Explosives are investigating the crimes along with the Austin Police Department.