Federal and local officials in Austin on Monday described a "change" in the methodology of whoever is behind a series of apparently connected blasts in the Texas capital. Austin's mayor says growing anxieties are "legitimate and real" following the fourth blast in the city this month.

A blast occurred Sunday night in a neighborhood far from the sites of the three package bombings in Austin this month. Police believe the latest explosion, which injured two men, is related to the first three, Austin Police chief Brian Manley said Monday. Manley said investigators have seen "similarities" between the device that exploded in the latest incident and the three other blasts, though a tripwire was used in the most recent explosion.   

"We are clearly dealing with what we expect to be a serial bomber at this point," Manley said.    

The victims, 22 and 23, are stable but have received "significant" injuries, Manley said. The injured men were riding or pushing bicycles when the explosives detonated, unlike the first three attacks, in which package bombs were left on people's doorsteps.

Manley said investigators have seen a "change" in the suspect's methodology. While the first three attacks appeared to be targeted, the fourth device was sitting next to a fence with a tripwire that could have been activated by anyone who came across it, he said. 

Fred Milanowski, special agent in charge of the FBI Houston division, said the most recent device was "more sophisticated" than the others.

Officials urged residents to stay away from any package, box or bag that looks suspicious -- or any item that looks out of place-- and to call 911.

Manley said authorities still don't know the suspect's motive, and whether it could be related to hate or domestic terrorism. Manley said persons of interest have emerged as tips have come in, and said they continue to investigate a "few" of those, but investigators have not identified a suspect.

Manley appealed publicly to whoever is behind the bombings, asking them to get in touch with police. A law enforcement official tells CBS News that behavioral analysts came up with the suggestion of reaching out to the suspect in a public way and that Manley was not responding to a message.

ATF and FBI were conducting a post-blast investigation and the area had been rendered safe, but will remain locked down until the afternoon as investigators process the scene. Manley appealed to the residents of Travis Country, where the latest blast occurred, to turn over any surveillance video they may have.

Manley said Monday that both men who were injured in Sunday night's explosion in the southwestern Austin neighborhood are white, unlike the victims in the three earlier blasts, who were black or Hispanic.

Officials are offering a combined $115,000 reward for information that leads to an arrest.