HOUSTON, March 18 (Xinhua) -- Two men were injured in an explosion on Sunday night in Austin, capital city of the U.S. state of Texas, according to local police and medical services, the fourth such attack in one month.

Brian Manley, Austin police chief, said in a press briefing that the police had confirmed one explosion in Travis County, 16 km southeast of downtown Austin, and the investigation will go through the night.

He urged the residents in the immediate area to stay inside their homes until further instruction from officers.

According to the Emergency Medical Services of Travis County, two men in their twenties had been transported with undetermined but not life-threatening injuries.

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

Police have not yet discovered a motive, but have not ruled out the possibility of hate crimes.

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

The first bomb exploded on March 2 while a man picked up a package outside his home in north Austin. Ten days later, two more bombs went off in the eastern part of the city.

Manley said it's not clear if Sunday's explosion is related to the previous incidents.