"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.
Authorities have not said whether that blast was also caused by a package bomb or if the victim, like those hit in the two confirmed bombings, is black.
The festival began Friday in downtown Austin and ends Tuesday.
In both cases, which occurred March 2 and Monday in Austin homes about 11 miles apart, packages were left on a doorstep during the night and residents found them when they first walked outside in the morning. Later that day, a 75-year-old woman was injured in a separate incident when she did the same.
The blasts occurred far from the heart of South By Southwest, which happens mostly in downtown Austin.
The police chief urged festival attendees to "be aware of what's going on".
Special Agent Michelle Lee, a San Antonio-based spokesman for the FBI, said the agency responded to both events and was assisting Austin police, who were leading the local investigation. He also said police are looking at any link between the three victims.
Neighbors say they are shocked by a second explosion at an Austin home on Monday. The second bomb severely injured an elderly woman. The woman in her 70s injured in that explosion was transported to an area hospital. It's not clear how the package got to the victim's home on March 2. A 17-year-old boy died from his injuries in the blast.
"We don't know what the motive behind these may be", he said.
That woman's injuries were not life-threatening, he said.
Later Monday a second explosion was reported in the 6700 block of Galindo Street in East Austin. "We believe the possibility that [these incidents] are related exists".
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. The request led to several calls to authorities, Manley said in the afternoon conference, but none uncovered anything suspicious.
Authorities had said they believe the first two explosions are linked.
The US city of Austin has been rocked by three parcel bombs in the last 10 days, two of which exploded on Monday, police said.
"Police Chief Brian Manley said Monday that investigators hope to collect surveillance video from nearby homes to identify a suspect". A 17-year-old male and an "adult female" found a package on their doorstep and opened it inside their kitchen.
Police Chief Brian Manley has warned Austin residents no to open their mail, following dual mail bombings in the Texas city on Monday. This compared to only two received the Monday before, the Austin American-Statesman reported.