What’s going on?

Residents of a southwest Austin neighborhood were told to stay inside early Monday after a fourth explosion in less than a month hit Texas’ capital, injuring two men and stoking fears in a city already on edge.

The latest blast occurred Sunday just after 8:30 p.m. local time (9:30 p.m. ET). Two bicyclists who are believed to be in their 20s sustained injuries that were not life-threatening, officials said.

The two men came across a suspicious device on the side of the road, Austin Police Chief Brian Manley said during a late-night news conference.

Unlike the prior explosions, the one Sunday may have been triggered by a tripwire, according to Manley.

“We will not be able to send school buses into the neighborhood on Monday,” he said. “In addition to that, we’re going to ask the residents in the Travis County neighborhood to stay in your homes tomorrow morning and give us the opportunity to process the scene once the sun comes up.”

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Asked Sunday whether the bombings were racially motivated, Manley said it was possible.

Police believe the two earlier bombings were “meant to send a message,” though Manley didn’t say what that message was during a news conference earlier Sunday.

The fourth explosion came just hours after officials made a direct appeal to the person responsible to contact them.

In that news conference, Manley said that he hoped the bomber was watching and would “reach out to us before anyone else is injured or killed.”

The plea came as local and federal authorities increased the reward for information leading to a conviction to $100,000, Manley said. Texas Gov. Greg Abbott was also offering $15,000.

“We don’t have any evidence,” Manley said. “What we know for certain is: We have three victims that are victims of color, and we have three package bombs that have exploded on the east side of Austin,” where many of the city’s minority residents live.