Manley said Mason's mother, who was also injured in the attack near Martin Luther King Jr.
The first of Monday's attacks killed a 17-year-old boy and wounded a 40-year-old woman, both of them black. The woman is hospitalized.
One theory that's being explored is that the explosions were racially motivated. The devices were essentially pipe bombs rigged to explode upon opening, the source said. Manley said anyone receiving a package they were not expecting should call 911.
Police later said they believe the two incidents are related.
There have not been any other type of package attacks, but police are still asking residents across Austin to be on the lookout and not open any unexpected or unmarked packages. He said that the police department received over 150 calls about suspicious packages, as of 5 a.m. March 13.
"It shook my house and it shook my body", Isaiah Guerrero told CNN affiliate KXAN of the explosion that injured the elderly woman.
"I don't want to give the package to my daughter and something happens, and I would regret it for the rest of my life", he told the station. He also was a longtime pastor at Wesley United Methodist Church, one of the city's oldest historically black churches. Dixon is friends with the grandfather of the teen who was killed Monday, according to the newspaper.
Austin Mayor Steve Adler, speaking Monday at a news conference, said, "This is still a safe city".
"Under no circumstances should you touch them, move them or handle them in any way", he said.
Police initially believed the March 2 incident was an isolated event, Manley 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. Manley said in both cases, the suspect delivered the packages during the nighttime, and the victims found them in the morning on their doorstep. Manley said a motive in the two incidents remains unclear, but the victim in the first and second explosion are African American and they can not rule out that the incidents were hate crimes. A few hours later, a third bombing injured a 75-year-old Hispanic woman. When she picked up the package, it detonated.
Austin police said the three explosions that have taken place within the past two weeks are connected.
The maker or makers put " these bombs together; they're transporting them to their intended location; and then actually ... detonating them", Manley told KXAN on Tuesday.
Texas Attorney General Ken Paxton urged all Texans to be cautious.
So far, the sources said that no family connections or obvious other ties have been found to connect the victims to each other. Some have changed their habits when it comes to handling packages.
"I took a walking stick and from behind my door, I cautiously tumbled the box over to bring up the label, where I could verify it was addressed to me and from the expected shipper", Mathis said.
Tina Sherrow, a retired agent for the Bureau of Alcohol, Tobacco, Firearms and Explosives, said the materials to build such bombs are commonly available at hardware stores or online, and that police have been mum on details because the perpetrators may be watching media coverage.
Texas Gov. Greg Abbott is offering a $15,000 reward for information leading to the identification and arrest of the person or persons involved in the deadly package blasts.
"We are not ruling anything out at this point", said Manley, who said the intended targets were not clear since multiple people live in the homes where explosives were placed.