(CNN) - An explosion that injured two men Sunday night -- the fourth package explosion in Austin, Texas, this month -- could have been triggered by a tripwire, said Austin Police Chief Brian Manley.
Authorities are working under the belief that the latest incident is connected to the previous three explosions in the city, Manley said. At this point, information is preliminary, he said early Monday morning, and police have yet to fully process the scene.
Manley said "it's very possible" that the device "was activated by someone either handling, kicking or coming into contact with a tripwire that activated the device."
The package on Sunday was left on the side of the road. The two injured men were either biking or pushing bicycles when the explosion occurred. Police are waiting for daylight to process the scene. Residents living near the scene have to stay indoors until 10 a.m. Hours before the latest blast, the chief appealed directly to whoever was responsible for the bombs.
Chief advises further caution
The two men, in their 20s, were taken to a hospital with serious but non-life threatening injuries, authorities said. Both patients are in good condition, a spokesperson at St. David's South Austin Medical Center told CNN.
The circumstances of Sunday's incident were a little different from previous explosions, the police chief said.
"We're not believing that this was similar to previous ones, as in packages left on doorsteps. But instead, this was some type of suspicious package that was left on the side of the road, that detonated and injured these two men," Manley said.
The men had been biking or walking their bicycles in southwest Austin when the explosion happened.
"What we do understand now, is that the possibility exists this device was triggered in a different mechanism -- that being a tripwire," he said.
Police and the FBI responded to the scene Sunday.
The latest explosion comes less than a week after police said three previous package explosions -- in a span of 10 days -- were connected. Those explosions killed a man and a teenager, and injured two others.
The victims in those three explosions were African-American or Hispanic. Police have not yet discovered a motive, but have not ruled out the possibility the bombs could be hate crimes.
Police are working under the belief that the explosions are related. Manley said they'll get a better idea with a post-blast analysis and examination of the device components.
In the meantime, Manley told residents not to touch or go near anything that looks suspicious.
On Monday morning, he warned: "We now need the community to have an extra level of vigilance and pay attention to any suspicious device -- whether it be a package or a bag, a backpack -- anything that looks out of place. And do not approach items like that."
Resident: 'It's concerning'
Stan Malachowski, who lives about half a mile away, said he heard a loud explosion.
"It was loud enough to hear inside of our house with our windows and door shut. Again, airplanes go by and cars backfire so we didn't think much of it," he told CNN affiliate KXAN.
"This is a quiet neighborhood. It's a family neighborhood. It's concerning."
Police warned residents in the immediate area to stay inside their homes until at least 10 a.m. All three entrances to the neighborhood have been blocked off.
Regents School of Austin, a nearby private school, will open two hours late Monday for "a complete security sweep," it said in a statement.
The Austin Independent School District announced that it would not be able to send school buses into the affected Travis Country neighborhood Monday, because of police activity. "Any tardies or absences due to this situation will be excused," the district posted on Facebook.
Many in Austin have been on edge since the bombings, as some residents of color in the Texas capital say they feel under threat.
The reward for information leading to the arrest of the person or people responsible for the three explosions increased to a total of $115,000, authorities announced earlier Sunday. Hours before the most recent explosion, the police chief appealed directly to whoever was responsible.
"We believe that the recent explosive incidents that have occurred in the city of Austin were meant to send a message," Manley said in a news conference.
"We hope this person or persons is watching and will reach out to us before anyone else is injured or anyone else is killed out of this event," he added.
Officials have urged residents to call police with any tips they may have, even if the information seems to be "inconsequential."
South by Southwest, the huge film, interactive and music festival, wrapped up Sunday, but a bomb threat was made against it Saturday that resulted in the cancellation of a concert featuring The Roots.
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