The fourth bomb explosion this month rocked Austin overnight, injuring two men and spreading fear throughout the Texas state capital. Hours prior to the explosion, Austin Police Chief Brian Manley urged the suspect or suspects behind the “serial bombings” to turn themselves in.

According to police, the latest bomb was triggered by a tripwire system. At a news conference, Manley said the latest bombing occurred when the two victims either stepped on or kicked a tripwire as they were walking in the neighborhood. The tripwire set off the device, which was left on the side of a residential road near a fence, at 8:32 p.m., Sunday, police said.

Investigators have found similarities between the latest device and three previous bombs, which were stuffed inside packages and left on residents’ doorsteps.

“Based on the preliminary review, we have seen similarities in the device that exploded last night and the three others,” Manley said. “We are clearly dealing with what we expect to be a serial bomber at this point.”

The police chief said the use of a tripwire is more sophisticated compared to the bombs that exploded on March 2 and March 12, indicating that those involved have “a higher level of sophistication, a higher level of skills” than previously suspected.

Global think tank Stratfor Threat Lens said, “The device’s success, despite significantly different design, further suggests that the bomb maker behind these attacks is an accomplished one, and has likely to have received some training, perhaps as a military or police explosive ordnance disposal technician.”

Fred Kolanowski, the agent in charge of the Alcohol, Tobacco, Firearms and Explosives office in Austin, told ABC News that the terrifying aspect of using a tripwire is that the victim triggering it could be anyone, including children in the residential area. He said the wire could be either a filament or fishing line that is hard to spot.

Police issued a “safety alert” in the wake of the bombing and advised all residents within a half-mile radius to stay in their homes until 10 a.m. “Stay inside your home until we have had a chance to deem this neighborhood safe,” Manley said at a news briefing Sunday.

The Austin public school district says it couldn’t send buses to the affected neighborhood Monday because of police activity. Police confirmed school buses would not be able to access the Travis Country neighborhood where the explosion took place and said “any tardies or absences due to this situation will be excused.”

The Travis County neighborhood remained on lockdown until 2 p.m., with residents being told to stay indoors as authorities investigated the pieces of evidence strewn across a wide area.

Unlike the first three bombings, the latest explosion occurred in a predominately white part of town, with both victims being white. The victims of the first blasts were black and Hispanic. However, police say that race could still be a factor.

“We’ve said from the beginning that we’re not willing to rule anything out just because when you rule something out you limit your focus,” Manley told ABC. “So this does change the concerns that we had initially although we have still not ruled it out until we understand what the ideology and motive is behind the suspect or suspects.”

Federal and state authorities have given additional resources to the investigation. There are currently some 500 federal agents in the Austin area working to find the bomber. Texas Governor Greg Abbott announced the release of $265,500 in emergency funding for the Austin Police Department and the Texas Ranger Response Team to purchase seven portable X-ray systems that can be used to provide “clear visual evidence for rapid assessment of a package’s safety,” the governor’s office said.

Houston and San Antonio have also sent teams to help with the investigation.

The fact that the bombing was seemingly random has further placed Austin’s residents on edge. The initial suggestions that the bombings were racially motivated is in doubt. Since the initial explosion, the Austin Police Department has gotten 735 reports of suspicious packages, and investigators have logged 236 interviews and checked out 435 leads, according to Police Chief Manley.