An Austin teenager was killed and several others injured Monday when two packages left on doorsteps exploded, marking the second and third deadly package explosions in the Texas capital in two weeks.

Austin Police Chief Brian Manley said the three incidents displayed enough similarities to lead police to investigate them together. Because two of the victims were African Americans and one was Hispanic, Manley said, investigators also were viewing the attacks as possible hate crimes.

His department is working with the FBI and the Bureau of Alcohol, Tobacco, Firearms and Explosives to determine whether the three bombs were similar and to identify the person, or people, who built them.

"We are not going to tolerate this in Austin," Manley said while briefing reporters on the explosion. "Every stop will be pulled out."

Texas Gov. Greg Abbott offered a $15,000 reward for information leading to the arrest of the person who planted the bombs.

"I want to assure all Texans, and especially those in Austin, that local, state and federal law enforcement officials are working diligently to find those responsible for these heinous crimes," he said.

In all three cases, packages were left on a doorstep during the night and residents found them in the morning.

The first incident occurred March 2 in a home in northern Austin. Anthony Stephan House, 39, was killed by the blast, but his death was not initially ruled a homicide because it was an "isolated incident."

After two more explosions on Monday, House's death was reclassified as a homicide and added to the broader investigation.

The first call Monday came at 6:44 a.m. A 17-year-old male and an "adult female" found a package on their doorstep and opened it inside their kitchen. Neighbors called police after hearing an explosion inside the house.

The teen died in the hospital. The woman received minor injuries.

"The damage is significant, and there's a lot of evidence that needs to be collected," Manley said.

As investigators combed through the scene, residents from south Austin called police about another explosion inside a house. Police responded and found a 75-year-old Hispanic woman who also had found a package outside that exploded when she opened it.

Manley said she was facing "life-threatening injuries."

Manley said his office still didn't know the motive behind the attacks. He said his office has not ruled out terrorism.

"We're looking at any possible avenues in these two cases," Manley said.

Manley urged Austin residents to call police if they find any package on their doorstop that gives them "cause for concern."

"If you find any suspicious packages on your front porch at your residence, do not handle them but instead call 911 and let us come out and take a look at those packages and ensure that they are safe," he said.

Manley said the U.S. Postal Service confirmed that its carriers had not delivered the package in Monday's explosion. And while police are checking with other delivery services, including Amazon, UPS and FedEx, Manley said it appeared the packages were not delivered through traditional services.

Manley said federal and local investigators would spend the day Monday analyzing the latest explosions and trying to piece together how the bombs were built.

He said the timing of the attacks is complicated by the fact that the city is hosting the SXSW conference, with business, music, film and technology leaders spread out at events throughout the city.

"Enjoy yourself, have a good time," Manley said. But "be aware, look for things that are suspicious."

This map shows three reported explosions in the Austin, Texas area. Officials are warning residents to not open any unknown packages.