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Chicago (AFP)

Two people were injured late Sunday in an explosion at a home in Austin, Texas, the fourth mysterious bombing in the city this month, police said.

The blast came just hours after police made a direct public appeal to the unknown bomber or bombers suspected of other recent bombings.

An exploding package killed a 39 year-old man on March 2. On March 12, one exploding package killed a 17-year-old boy and another separate one injured a woman.

After the latest blast, the county Emergency Medical Service said on Twitter that they responded to reports of an explosion around 8:30 pm (0130 GMT Monday), and two men in their 20s were rushed to a hospital with serious injuries.

Authorities were investigating the previous bombings as possible hate crimes as the two men killed were African-American, and a Hispanic woman was seriously injured.

Police did not mention the race of the Sunday victims.

"Stay inside your home until we have had a chance to deem this neighborhood safe," Austin interim Police Chief Brian Manley said at a streetside news conference after the blast.

"That will not be, at a minimum, until daylight, given the darkness and the size of the area that we want to go in and check to make sure again that this neighborhood is safe."

Because of the darkness "we have not really had an opportunity to really look at this blast site to determine what has happened," Manley said.

- $115,000 reward -

Police believe the earlier attacks were related. All of the cardboard packages were hand-delivered, not sent through the mail, and the bombs were built with household items available at hardware stores.

At a later press conference early Monday, Manley said that the Sunday explosive may have been activated by a trip-wire, unlike the previous bombs.

It is "very possible" that it was "activated by someone either handling, kicking or coming in contact with a tripwire that activated the device," Manley said.

"That changes things," he said. "We now need to have an extra level of vigilance and pay attention to any suspicious device, whether it be a package, a bag, a backpack, anything that looks out of place."

Manley told reporters that the two victims were either riding their bikes or pushing them when a suspicious package on the side of the road detonated.

He told the Austin-American Statesman that investigators are operating under the assumption that the attack is connected to the three prior blasts.

A task force of hundreds of agents is working on the case, including criminal profilers and experts from the Federal Bureau of Investigation (FBI) and the Bureau of Alcohol, Tobacco and Firearms (ATF).

At an earlier press conference Sunday, Manley addressed the unknown bomber or bombers directly.

"We want to understand what brought you to this point, and we want to listen to you," he said.

Police also said they were increasing the reward offered for information leading to an arrest, bringing the total city and state bounty money to $115,000.

© 2018 AFP