HOUSTON, March 19 (Xinhua) -- There are "similarities" in the latest Austin bomb and three other previous bombs, Austin police chief Brian Manley said in a press conference Monday morning in Austin, Texas, the United States.

"Based on the preliminary investigations we have done at this time we have seen similarities in the device that exploded here last night and other three devices that had exploded in Austin starting on March 2," Manley said.

He also mentioned that the big difference of the explosion last night was that a "trip wire" was used in the device.

ATF (Bureau of Alcohol, Tobacco, Firearms and Explosives) special agent in charge Frank Milanowski explained that a trip wire is a victim activated switch that uses some kind of wire. "When pressure puts on the wire, it activates detonated device," he said.

Due to the sophistication of a trip wire, Manley asked people not to approach any suspicious items, such as a bag, suitcase or box.

He also asked people living in the neighborhood of the explosion to help police for the investigation.

"If you have video surveillance on your house, we want to get your video footage so that we can have that analyzed and identify any potential suspicious persons, vehicles, or anything that may be interest to this investigation," he added.

Two men aged 22 and 23 were injured in an explosion Sunday night about 16 kilometers southwest of downtown Austin. Investigations showed that they were injured by an explosion device on the roadside near a fence.

The explosion came just days after three package bombs were delivered to Austin residents, killing two and injuring several others.

Police have not yet discovered a motive, but said it is too early to tell if these bombs are hate crimes or domestic terrorism.

Just this weekend, Austin police added 50,000 U.S. dollars to the reward for information that leads to an arrest in the bomb cases, which brought the total reward to 115,000 dollars.