Sunday's trip-wire bomb was likely a deliberate attempt by the bomber to let Austin police and the more than 500 federal agents on the case know he had shifted tactics, said Burton, who was a counterterrorism agent with the U.S. State Department from 1985 to 1999.
And they warned that they are dealing with a highly-skilled assailant, with Austin police chief Brian Manley saying: 'If this explosion last night was the result of a bomb that was utilizing tripwire technology, that is showing a different level of skill, above what we were already concerned that this suspect or suspects possess.
"We are clearly dealing with what we expect to be a serial bomber at this point, based on the similarities", Manley said at a press conference this morning, adding that authorities are not ruling out that the bombings are "domestic terrorism or hate-related".
"What we do know is that the individual or individuals involved in this, these suspects, they do have a certain level of skill to be able to construct a device like this and then deliver that device to your target without having it explode, either during construction, or during delivery, does take a certain level of sophistication", Manley said.
The fourth explosive detonated shortly after police appealed to the person responsible for the three previous bombings to turn themselves in.
Both men were African-Americans.
Frederick Milanowski, the special agent in charge for the Bureau of Alcohol, Tobacco and Firearms, says the latest bomb is "more sophisticated" because it used a tripwire.
Austin Mayor Steve Adler told Good Morning America on Monday there was "not enough data" to be able to understand what the motives were. The prior victims were people of color, which had set off fears that maybe the bombings were racially motivated.
Sunday also was the final day of the South By Southwest music festival, which draws hundreds of thousands to Austin every March.
Police asked anyone with surveillance cameras at their homes to come forward with the footage on the chance it captured suspicious vehicles or people.
The Austin public school district says it couldn't send buses to the affected neighborhood Monday because of police activity.
"When they said that, it was nearly like they had stopped at that point", Stein says.
A 19-year-old College Student Has Gone Missing in Bermuda
Joseph's University says they are in touch with Mark's family and Bermuda authorities and continues to pray for his safe return. According to the university, Dombroski, a member of SJU's men's rugby club, was reported missing in Hamilton, Bermuda.
The attacks took place in the east of a city in which the Interstate 35 freeway that runs through downtown has historically acted as a dividing line between predominantly white areas to the west and mainly black and Hispanic neighbourhoods on the other side.
Now, "it's obvious that you have to be alert".
US code defines domestic terrorism as life-threatening acts that appear meant to intimidate civilians; influence government policy through intimidation or coercion; or affect the conduct of government through mass destruction, assassination or kidnapping.
"It wouldn't be a typical household that would have all these components, but I would say that all the components are commercially available", he said.
Manley said "it's very possible" that the device "was activated by someone either handling, kicking or coming into contact with a tripwire that activated the device".
It also urged students to report suspicious items to authorities.
Fear mounted across Texas' capital on Monday after the fourth bombing this month - a blast triggered this time by a almost invisible tripwire, demonstrating what police called a "higher level of sophistication" than the package bombs used in the previous attacks. Those blasts killed a man and a teenager, and wounded two others.
Police are working under the belief that the explosions are related.
Manley said it is not believed that this third package was left by an official delivery service, and they now do not have any suspect or vehicle descriptions at this time.
Authorities have cordoned off the neighborhood where the bomb went off Sunday night and have warned residents to remain indoors while officers check for anything suspicious.
"We have had persons of interest and we continue to look at a few, but we have not identified a suspect or suspects at this time", Manley said Monday.
- Trump bans Venezuelan cryptocurrency in the US
- Parkland shooting surveillance videos released
- Loyola-Chicago sports its winning Jean, again
- American Idol: Audriana Bolton Sings Mariah Carey In Daisy Dukes
- Egypt's Mohamed Salah Leads Lionel Messi In European Golden Shoe Race
- Cigna to spend about $52 billion for Express Scripts
- India Issues Thirteenth Note Verbale to Pakistan Over Harassment of Diplomats
- Kelly seeks to reassure White House staff, pledges 'no immediate personnel changes'
- Spring Snow: Wintry mix starts Tuesday, but storm looks more likely Wednesday
- Russia Calls British PM's Allegations Over Poisoning 'Insane'