AUSTIN - Texas police were hunting for a mysterious “serial bomber” Monday after an explosion in the state capital Austin - the fourth this month - left two young men seriously injured and the city on edge.

Police said Sunday night’s blast was connected to three previous bombings in Austin and the bomber used a tripwire in the latest attack, showing a “higher level of skill.” “We’re clearly dealing with what we expect to be a serial bomber at this point,” Austin police chief Brian Manley told reporters.

But Manley said police have been unable to determine a motive for the bombings which have killed two people in the city of nearly one million people and injured another four. “Is this terrorism? Is it hate-related?” Manley asked. “As we said from the very beginning, we were not willing to classify this as terrorism, as hate, because we just don’t know enough.”

In the earlier bombings, two African-American men were killed by packages left on their doorsteps.

, raising the possibility of a racially motivated crime. A 75-year-old Hispanic woman was also injured in a blast.

But Manley said the latest bombing seriously wounded two white men aged 22 and 23 as they walked along a sidewalk or on the road in a quiet residential neighborhood of southwest Austin.

He said it appeared “random” and was triggered by a tripwire.

“What we have seen now is a significant change from what appeared to be three very targeted attacks to what was last night an attack that would have hit a random victim that happened to walk by,” he said.

“We’ve definitely seen a change in the method that this suspect or suspects are using,” the police chief said.

Manley said the use of a tripwire also means police are dealing with someone who “shows a higher level of sophistication, a higher level of skill” than initially believed.

“With this tripwire, this changes things,” agreed FBI special agent Christopher Combs. “It’s more sophisticated, it’s not targeted towards individuals.

“We’re very concerned that with tripwires a child could be walking down a sidewalk and hit something,” Combs said.

Manley, the police chief, appealed to the bomber to contact the authorities and to the public to report anything suspicious.

“I will reach out to the suspect or suspects and ask that you contact us, ask that you reach out to us, communicate with us so that we can put this to an end,” Manley said. “There are innocent people getting hurt in this community and it needs to come to a stop.

“People need be vigilant, pay attention,” he added. “Pay attention to your surroundings.”

Police said they were increasing the reward offered for information leading to an arrest, bringing the total city and state bounty money to $115,000.

“We need every tip, every piece of information, however inconsequential you may think it is,” Manley said.

An exploding package killed a 39-year-old African-American man, Anthony House, on March 2. A 17-year-old African-American man, Draylen Mason, was killed on March 12 and the Hispanic woman was critically injured the same day.

All of the cardboard packages were hand-delivered, not sent through the mail, and the bombs were built with household items available at hardware stores.

A task force of hundreds of agents are working the case, including criminal profilers and experts from the Federal Bureau of Investigation and the Bureau of Alcohol, Tobacco and Firearms (ATF).

Austin Mayor Steve Adler said the manhunt for the bomber was the “highest priority.”

“We have some of the best law enforcement folks around dealing with this,” Adler said on ABC’s “Good Morning America.” “There’s an army of federal agents. We have state resources.

“We’re going to find out who is responsible for this and we’re going to stop it,” he said.

Since the bombings began, city police have responded to some 700 suspicious package calls, according to the Austin-American Statesman newspaper.