AUSTIN, Texas—A 15-year-old aspiring rapper named Isaiah Guerrero was in his bedroom recording a track on Monday morning when a bomb exploded outside his window.

“I was flowing,” he said, “I was freestyling, and I felt it. My whole body shook.” He climbed to his roof and saw smoke rising from a nearby house. Paramedics arrived and carried an elderly woman out on a stretcher and transported her to the hospital, where she remains in critical condition, police said, the victim of a mysterious parcel bomber terrorizing Austin’s impoverished East Side, whose residents are primarily African-American and Hispanic.

It was the second attack of the morning and the third in 11 days. The Austin Police Department is now facing tough questions about why it didn’t warn the public after the first incident, on March 2, when an improvised explosive device killed an Austin resident on Haverford Drive.

Although any kind of bombing is unheard of in this low-crime city, police did not initially treat the March 2 incident as a homicide. Instead, they cast suspicion on the victim, a 39-year-old African-American man named Anthony Stephan House.

“We can’t rule out that Mr. House didn’t construct this himself and accidentally detonate it,” APD Assistant Chief Joseph Chacon told reporters at the time.

He added that there was no continuing threat to the public. “I do not believe that we have someone going around leaving packages like this,” he said.

On March 5, APD released a “suspicious death” bulletin that said, “we believe this is an isolated incident and that there is no continuing threat to the community.”

Facing questions from reporters after the second and third bombings on Monday, Mayor Steve Adler defended the city’s failure to warn residents sooner.

“There was nothing about the first incident that would have indicated in any way that it was anything but an isolated incident,” he said. “And just as soon as it appeared that it was not, the public was informed right away.”

Asked whether it was a mistake to treat House as a suspect, or whether House had been definitively cleared of wrongdoing, Adler declined to answer, citing the ongoing nature of the investigation.

“ We can’t rule out that Mr. House didn’t construct this himself and accidentally detonate it. ” — APD Assistant Chief Joseph Chacon

House played football for Pflugerville High School, graduated from Texas State University in 2008, and worked as a project manager for Texas Quarries, according to his Facebook page. Friends and neighbors told reporters he was married with an 8-year-old daughter and was president of his homeowners’ association.

On Monday, APD reclassified his death as a homicide, warned the public not to open any unexpected packages, especially hand-delivered or odd-looking ones, and said it is working with the FBI and the ATF to hunt down what would appear to be a serial bomber on the loose.

It was around dawn on Monday that Jay Brewer, 52, heard a loud noise outside his home on Oldfort Hill Drive.

“It sounded like a car crash,” he said. “I never would have imagined it was a bomb.”

Half a block north, a 17-year-old African-American boy was killed and his mother seriously injured by a device in a package left on their stoop. Authorities have yet to release the young man’s name.

The second bomb went off about five hours later, a few miles to the south, on Galindo Street in the Montopolis neighborhood, one of Austin’s poorest and least gentrified areas.

Neighbors identified the elderly woman who was injured in the blast as Esperanza Moreno, 75, and were at a loss as to why she might have been targeted.

“I can’t explain why anyone would want to do this to her,” said Rosaria Lopez, who was prevented from getting to her house by a police cordon. She said she would often visit Moreno to chat and exchange recipes. “She’s a good person.”

Police said they have yet to identify a common thread among the victims. The fact that all were African-American or Hispanic has fueled speculation that some kind of racist Unabomber is behind the attacks, a notion some Montopolis residents found hard to believe.

“It wasn’t no hate crime,” said Brandon Rendon, 27, sitting under his carport on Lawrence Street. “It was a random act.”

But no one could remember anything remotely similar ever happening in this homely but verdant neighborhood, where chickens run loose and people barbecue in the streets, far from the hordes of well-dressed scenesters currently descending on downtown Austin for the annual South-by-Southwest conference.

“You get shootings and stabbings in Montopolis,” Guerrero said. “Not bombings.”

Police have asked anyone with information on the attacks to come forward, and dispatchers spent the day fielding dozens of suspicious-package calls. The governor of Texas, Greg Abbott, said the state’s Criminal Justice Division is offering a reward of up to $15,000 for information leading to an arrest.