An explosion, possibly triggered by a tripwire, tore through a quiet southwest Austin, Texas, neighborhood Sunday night, leaving two men seriously injured and heightening concerns that a serial package bomber was targeting the city’s residents since the beginning of this month.

Hours after Austin’s police chief urged the suspect in the recent bombings to turn themselves in to the authorities, local and federal emergency units responded to the fourth blast, shortly before 9 p.m. EDT on Sunday, near the Greenbelt just north of the Y in Oak Hill. Several streets were closed, residents warned to remain inside and a massive law enforcement contingent comprising Austin police and FBI agents was brought to the neighborhood.

Some neighbors near the area of the explosion told media outlets they were informed the explosion was the result of a tripwire, but the authorities have not confirmed any details.

"There have been reports in the media that this device was triggered by a tripwire, and we are here to say that that is a possibility. We understand that those reports are out there and it is very possible that this device was a device that was activated by someone either handling, kicking or coming into contact a tripwire that activated the device," Austin interim police chief Brian Manley said.

Two men believed to be in their 20s were hospitalized with serious injuries, but officials later stated their condition was stable.

"We do believe based on what we've seen that this is a bomb," Manley said and added police are "working under the belief that they [all four explosions] are connected" however, have not yet processed the scene.

Austin explosion Police and FBI officers guard the scene of an explosion in Austin, Texas, March 12, 2018. Photo: REUTERS/Sergio Flores

Manley said in a press conference late Sunday evening anyone with half-mile radius near the area of the explosion should stay inside and avoid going out until daylight (10 a.m. EDT on Monday). He added "school buses can’t be sent in Travis Country neighborhood. Tardies and absences will be granted."

The police chief said a backpack was found near the scene of the explosion and it was being searched as a suspicious item.

If Sunday’s blast is found to be connected to the three other bombings that killed two Austin residents and injured two others since March 2, it would then mark a geographic widening of the serial bomber’s targets, if the suspect is also discovered to be the same in all four explosions.

The first blast took place March 2 and the second and the third ones, 10 days later, on March 12.

The first three bombs were found east of Interstate 35 and hit black or Hispanic residents. In all three of those cases, the package bombs were planted overnight on the victims’ doorstep, police said.

The first two victims, 39-year-old Anthony Stephan House and 17-year-old Draylen Mason, were found to be connected to two prominent African-American families that had ties to an East Austin church and long histories of battling for racial justice and empowerment of the city’s black community.

The third bomb was said to have hit a Latina resident and her mother in Montopolis. Police have not yet found a motive but did not rule out the possibility the explosions could be hate crimes.

The reward for any kind of information leading to the arrest of the suspect or suspects responsible was increased to a total of $115,000, authorities announced Sunday.

Previously Gov. Greg Abbott's office offered $15,000, in addition to which the FBI, the ATF and the Austin Police Department are now offering a $100,000 for information regarding the blasts.