AUSTIN, Texas – The reward for the Austin serial bomber has topped $100,000. Moreover, Austin police sent a message Sunday to the person behind a string of deadly bombings that have rocked the city: We are listening and want to talk.

More than a month has passed since the first bomb went off in the Texas capital. Since then, there have been two other bombs and hundreds of tips to police, but so far no suspects or arrests, leaving the city on edge, reported USA Today. (See update below regarding a possible trip wire explosion that occurred Sunday night.)

Austin Police Chief Brian Manley announced the reward was increased to $100,000 for information that leads to an arrest in the case. That, along with an award from the governor’s office, would bring the total reward to $115,000.

The chief pleaded during a Sunday afternoon news conference to those responsible.

“We hope this person or persons is watching and will reach out to us before anyone else is injured or anyone else is killed,” he said.

“We assure you, we are listening and we want to understand what brought you to this point, and we want to listen to you, so please call us,” he added.

Manley said investigators know the incidents were meant to send a message. However, they are unsure what it is. Furthermore, he said hundreds of officers are investigating, more than 400 leads are being followed, and more than 200 interviews completed. Yet detectives haven’t been able to piece together who is behind the string of deadly attacks.

According to the chief, this type of investigation and the number of resources on it is “unprecedented” in Austin’s history.

All three bombs were placed on doorsteps in plain cardboard boxes, so police have encouraged residents to call authorities if they see an unexpected box on their doorstep.

Two people were killed and two others were injured. Police initially raised the possibility of the bombings being race related because victims in the first two explosions were black. The third apparent target was Hispanic, so it’s not clear whether that pattern is intentional.

Manley said a motive is still unclear. As a result, detectives don’t want to limit the scope of their investigation.

The first package exploded on March 2, killing Anthony Stephan House, 39, when he picked up a package on the front porch of his northern Austin home.

The second deadly explosion occurred before 7 a.m. on March 12 inside a home in East Austin. Police said Draylen Mason, 17, brought the package inside the kitchen and was opening it alongside his mother when it detonated.

Consequently, Mason died, and his mother was hospitalized.

The third blast came a short time later in a neighborhood south of downtown Austin. A 75-year-old Hispanic woman was picking up a package on her front porch when it exploded, seriously injuring her.

Manley said she remains hospitalized with “life-threatening injuries” and authorities are not publicly identifying her.

Manley said investigators from the Bureau of Alcohol, Tobacco, Firearms and Explosives have examined all three explosive devices and found the materials used are highly dangerous and hard to keep stable.

He said investigators now know much more about how the devices were constructed, adding they pose an “ongoing hazard” to anyone in the area.

The bombings come as the city hosted the annual South by Southwest festival, which attracted global leaders in business, technology, music and film. Festival organizers issued a statement that they were “heartbroken” by the explosions and urged attendees to keep an eye out for any suspicious behavior.

A concert at the festival was canceled Saturday evening after an unrelated bomb threat. That person was arrested but police say the suspect was not tied to the package bombs.

UPDATE:

Another Austin explosion — possibly triggered by a trip wire — injured two people on bicycles Sunday night, leaving police frantically working to determine if the blast is linked to a trio of package bombings that have gripped the Texas capital in fear.

The latest blast occurred around 8:30 p.m. Sunday night in a suburban neighborhood known as Travis Country. Investigators didn’t immediately confirm what caused it, reported Fox News.

Austin police Chief Brian Manley said the explosion may have been detonated by a trip wire, adding the blast was “activated by someone either handling, kicking or coming into contact with a trip wire that activated the device” as they walked or rode their bikes through the area.

“That changes things, in that our safety message to this point has been involving the handling of packages and telling communities ‘do not handle packages, do not pick up packages, do not disturb packages,'” Manley said at a hastily-arranged news conference. “We now need the community to have an extra level of vigilance and pay attention to any suspicious device, whether it be a package, a bag, a backpack, anything that looks out of place. Do not approach it.”

Manley said police were “working under the belief” the explosion was related to the three others, but investigators still had yet to process the entire scene. He urged people within a half-a-mile radius to stay in their homes until at least daybreak.

A witness speaking to FOX7 described hearing a “loud bang,” adding that it was “not a car crash, not gunshots but something terrible.”

Two men in their 20s suffered non-life threatening injuries in the blast, including one who had nails in his leg, according to KVUE-TV. South Austin Medical Center officials said the men were in good condition.

Sunday’s explosion was the fourth to rock Austin less than three weeks. However, the three previous blasts occurred on the eastside of the city.