Police suspect 'serial bomber' behind deadly Austin attacks

Four bombs that havegone off this month around Austin, Tex.,have similarities that leadinvestigators to believe they are the work of a serial bomber,police said on Monday.

The latest bombing on Sunday injuredtwo men andmay have been activated by a trip wire, a moreadvanced design than the previous explosions that were set offwhen victims handled packages that were left on doorsteps. Thosepackages killed two earlier this month.

"We are clearly dealing with what we expect to be a serialbomber at this point," Austin Police Chief Brian Manley told a newsconference. "We have seen similarities in the devices thatexploded here last night and the other three devices."

The men, one aged 22 and the other 23, suffered non-lifethreatening injuries and were taken to the hospital on Sunday after they came upon a suspicious device on the side of a roadin a residential neighbourhood, officials said.

Agents from the Federal Bureau of Investigation and U.S.Bureau of Alcohol, Tobacco, Firearms and Explosives were at thescene, the FBI said on Twitter.

Investigators are trying to identify the person or peoplebehind the three parcel bombs that exploded in three separate east side neighbourhoods, killing two African-American males andleaving a 75-year-old Hispanic woman fighting for her life.

The most recent blast happened in the southwestern Austin residential neighbourhood of Travis Country. The three previous bombings happened in residential neighbourhoods east of Interstate 35, which divides the city.

Police were warning people to be extremely cautious ifthey see something suspicious.

LauraMcGinnis, who lives in the Travis Country neighbourhood in southwestern Austin, said Monday that Sunday night's attack is frightening but that it hasn't personally affected her yet. However, she saidthe bombing and three others elsewhere in the city this month make her wonder why the bomber hasn't been caught.

AdamMcGinnis, who also lives in the neighbourhood, saidhe was reading on his back porch Sunday night when he heard what sounded like a large gun blast. He saidhe figured it must have been a transformer that blew and didn't realize it was a bombing until his wife told him Monday morning.

• TIMELINE |Massive investigation unfolding after 4 bombings in Austin

Austin police have said whoever was responsible was trying tosend a message and should contact authorities to explain any motive.

"We are not going to understand that [message]until thesuspect or suspects reach out to us to talk to us," Manley said.

Manley said police were also investigating the bombings aspossible hate crimes.

Police get hundreds of calls

The first bombing on March 2 killed Anthony Stephan House, a39-year-old black man. It ripped a hole in a home entrance walland damaged the front door.

A bomb last Monday morning killed Draylen Mason, a17-year-old African-American teenager and budding musician, andinjured his mother, who is in her 40s but was not furtheridentified. A few hours later, a third bomb injured the 75-year-old Hispanic woman, who has not been identified.

The victims in Sunday's explosion arewhite, Manley toldABC News on Monday morning.

Police have received more than 700 calls about suspiciouspackages since the three parcel bombs, but authorities have notfound any that posed a security risk, Manley said.

A reward of $115,000 US has been offered for informationleading to the arrest and conviction of the person or people responsible.