On Monday, the 17-year-old found a package outside his house and brought it into the kitchen, where it exploded, Manley said.
Manley said there is no known motive for the explosions, but he said local and federal agents are exploring whether the incidents could be racially motivated since the first two victims were African-American and the elderly woman is Hispanic.
Monday's second explosion - the cause of which was still being investigated - happened in the Montopolis neighborhood, near the airport and about 5 miles south of the day's first blast.
She is in the hospital in serious condition.
Police Chief Brian Manley said at a news conference Monday that investigators believe the deadly blast Monday is linked to a March 2 package bomb that killed a 39-year-old man in another part of the city.
"The world is just filled with insane people, that in their minds, the only crime is how you were born", says Jay Brewer, neighbor.
Investigators in Austin, Texas searched Tuesday for answers behind the string of explosive packages that detonated recently at homes around the city, describing the devices as sophisticated while struggling to identify who had sent them out or why.
The explosion happened hours after a package bombing a few miles away that killed a teenager and seriously injured a woman.
Police did not however confirm an immediate connection to the first two incidents - or whether the victim was black. The March 2 blast was initially investigated as a suspicious death, but is now viewed as a homicide.
"We do not know yet whether the victims are the intended targets", he said, adding that some homes had multiple residents and the bomber may have also targeted the wrong addresses.
In total, the three explosions have killed two people and injured two others.
That woman's injuries were not life-threatening, he said.
Investigators originally suggested that the attacks could constitute a hate crime because the victims were all black or Hispanic, but they now say they are looking at all possible explanations.
"I don't understand it, I just don't understand it", one resident said. The first Monday blast was reported at 6:44 a.m.in the 4000 block of Oldfort Drive and the second Monday blast was around noon in the 6700 block of Galindo Street.
The Bureau of Alcohol Tobacco and Firearms said a national response team would join Austin police in the investigation. "Under no circumstances should you touch them, move them or handle them in any way", he said. "Call 911 immediately if you receive something suspicious", he said on Twitter.
The FBI is helping Austin police in the investigation. "The ATF explosive specialists and their laboratory are quite adept at recovering evidence that frequently can lead to determining the components of the device". The Texas Department of Public Safety is involved as well, and a $15,000 reward has been offered.