March 19 (UPI) -- Two men were injured Sunday night when a package left on their doorstep exploded. It was the fourth such attack in the Texas capital this month.
"We are clearly dealing with what we expect to be a serial bomber at this point," Austin Police Chief Brian Manley said mid-morning Monday. "We have seen similarities in the device that exploded here last night and the other three devices that have exploded in Austin."
Police said the device that exploded Sunday night may have been triggered by a different mechanism: a tripwire. Investigators said the tripwire is the main difference between Sunday's attack and the others.
"The belief that we are now dealing with someone who is using tripwires shows a higher level of sophistication, a higher level of skill," Manley said, warning that residents should not approach any suspicious items like backpacks, boxes or suitcases.
"It is very possible that this device is a device that was activated by someone either handling, kicking or coming in contact with a tripwire activating the device," Manley said earlier. "Do not handle packages, do not pick up packages, do not disturb packages."
Manley also made a direct appeal to the bomber.
"I will reach out to the suspect or suspects and ask that you contact us, ask that you reach out to us. Communicate with us so that we can put this to an end," Manley said. "There are innocent people getting hurt in this community and it needs to come to a stop."
The police chief asked residents to instead call 911 to report suspicious packages.
A tripwire is activated when pressure is placed on it, by stepping on it or kicking it, officials said. Tripwires can be made from fishing line or a metal wire attached to the bomb.
Authorities said they performed another sweep of the area Monday morning and brought in specialists to conduct a post-blast investigation -- a team that includes more than 350 FBI special agents.
Manley encouraged Travis County residents with surveillance cameras at their homes to hand over their footage, hoping it might contain additional clues.
Sunday's explosion was the fourth mail package attack in Austin this month. Two men were killed in three bombings on March 2 and March 12. No arrests have been made.
Two men, ages 22 and 23, were injured by Sunday's bomb, but will survive.
Sunday's explosion occurred just hours after the FBI increased its reward for information to $100,000.
Texas Crime Stoppers is offering an additional $15,000 for the bomber's arrest and conviction.