AUSTIN, Texas (Reuters) - Police in Texas fear a serial bomber planted four powerful explosive devices that have killed two people and injured four others this month, raising fears in the state capital Austin of another attack.

Investigators said they have no clear idea what motivated the series of attacks, which began 17 days ago, and have publicly reached out to the bomber, urging him or her to contact them and explain what his demands are.

The first three devices were parcel bombs dropped off at night in front of homes on the city’s east side, with the fourth a trip wire device that went off in a west side neighborhood on Sunday. All four devices were similar, police said on Monday.

“We are clearly dealing with what we expect to be a serial bomber at this point,” Austin Police Chief Brian Manley told a news conference near the site of Sunday’s bombing.

Two men were injured on Sunday by the latest bomb, which police said may have been activated by a trip wire across a sidewalk. It was a more advanced design than the previous bombs.

The men, 22 and 23 years old, were taken to a hospital with what police described as “substantial” but not life-threatening injuries.

Manley said more than 500 federal agents were involved in the investigation, including from the Federal Bureau of Investigation and U.S. Bureau of Alcohol, Tobacco, Firearms and Explosives.

“There is an army of law enforcement folks on the scene right now,” Austin Mayor Steve Adler said on CNN. “I am confident that we’re going to find whoever is responsible for this and then we’re going to stop it.”

FBI agents have swept the relatively affluent neighborhood called Travis Country where Sunday’s bomb exploded and asked residents for home surveillance videos, residents said.

“It’s scary,” Thad Holt, a 76-year-old retiree, said in an interview, recalling that he and his wife had strolled near the bomb site about half an hour before the explosion. “It’s one of those things ... that happens elsewhere.”

Austin, with a population of nearly 1 million people, is home to the University of Texas and a plethora of tech companies and has been one of the fastest growing major U.S. cities.

MOTIVE UNKNOWN

The first bombing, on March 2, killed Anthony Stephan House, a 39-year-old black man. It ripped a hole in a home entrance wall and damaged the front door.

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A bomb last Monday morning killed Draylen Mason, a 17-year-old African-American teenager and budding musician, and injured his mother, who is in her 40s but was not further identified. A few hours later, a third bomb injured the 75-year-old Hispanic woman, who has not been identified.

Austin police have asked the bomber or bombers to contact them and explain the motivation behind the attacks.

“The person is actually trying to provoke fear and that is made worse by simply not giving information about why,” said Robert Pape, an international security expert and director of the University of Chicago Project on Security and Threats.

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“There has not been an effort from the attacker to get publicity, and quite a few attackers are interested in what they see as glory and attention,” Pape said in a telephone interview.

Chief Manley has said the bombings were being investigated as possible hate crimes, but cautioned that Sunday’s attack did not appear to have targeted specific people, and both victims were white.

A reward of $115,000 has been offered for information leading to the arrest and conviction of the person or persons responsible.