Last Updated Mar 19, 2018 7:08 PM EDT
AUSTIN -- Federal and local officials in Austin on Mondayin the Texas capital, describing a "higher level of sophistication" in the device that detonated Sunday than in the package bombs used in the previous attacks. Austin's mayor says growing anxieties are "legitimate and real" following the fourth blast in the city this month.
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 latest attack happened in the southwestern Austin neighborhood of Travis Country. That is far from the sites of the earlier bombings, which took place in residential neighborhoods east of Interstate 35 and killed a 39-year-old man and a 17-year-old boy and wounded two other people.
A Travis Country resident shared home surveillance video with CBS affiliate KEYE-TV, in which the Sunday explosion can be heard nearby.
Police at first suggested the bombings could be hate crimes since the first three bombs involved victims who were black or Hispanic, but the latest attack may have undermined that theory. The 22- and 23-year-old men wounded this time are white.
The victims 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.
Officials said the most recent device was "more sophisticated" than the others. Christopher Combs, the special agent in charge of the FBI's San Antonio division, told The Associated Press on Monday that investigators are looking for a dialogue with the bomber.
He notes that the stakes "went up a lot" on Sunday with the bomber's use of a tripwire. Authorities say the latest bomb was anchored to a metal yard sign near a hiking trail and equipped with a fishing line-thin tripwire.
Frederick Milanowski, the special agent in charge for the Bureau of Alcohol, Tobacco and Firearms, said at a news conference Monday that the wire or filament that detonated the bomb Sunday night would have been very difficult to see.
Combs, who is investigating the Austin attacks, says, "The bomber has obviously shown us that he has the ability to make more complex devices, to hurt more people, to be more random. And that's not good. That why we need to talk to the bomber about what is going on."
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.
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.
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.
Officials are offering a combined $115,000 reward for information that leads to an arrest.