ALVARADO, Texas — Ten people have been arrested on attempted murder charges in connection with a shooting outside of a Texas immigration detention center that wounded a police officer, where people in black military-style clothing fired at authorities in a “planned ambush,” a U.S. attorney said.

The officer was shot in the neck on Friday, the night of the Fourth of July, after reporting to the Prairieland Detention Center in Alvarado. He was treated at a hospital and released, the Johnson County Sheriff’s office said.

Before that, the group dressed in black began shooting fireworks, damaging cars and a guard structure, and writing words such as “traitor” and ”ICE pig.” All of that “seemed to be designed to draw U.S. Immigration and Customs Enforcement personnel outside the facility, and it worked,” Nancy Larson, acting U.S. attorney for the Northern District of Texas, said at a Monday night news conference.

Two unarmed corrections officers came out of the detention center to speak with the group. At some point, the Alvarado police officer arrived and was shot by someone who was in the woods, Ms. Larson said.

“Another assailant, who was across the street, nowhere near the corrections officers, shot 20 to 30 rounds at these unarmed corrections officers,” she said. “There was an AR-style rifle found at the scene” that was jammed, she said. A flag saying “Resist fascism, fight oligarchy,” and flyers with words such as “Fight ICE” also were recovered near the center.

The group fled. Sheriff’s deputies stopped seven people about 300 yards from where the officer was shot.

“Some were wearing body armor, some were covered in mud, some were armed, and some had two-way radios on them,” Ms. Larson said.

The shooting took place as President Trump’s administration ramps up deportations, which will be turbocharged by his massive agenda bill that became law last week. It included more spending on immigration enforcement.

Additional searches led to masks, goggles, tactical gloves, more body armor, weapons, spray paint and fireworks, Ms. Larson said.

“It was a planned ambush with the intent to kill ICE corrections officers,” she said.

The 10 were each charged with three counts of attempted murder of a federal officer and three counts of discharging a firearm in relation to a crime of violence. Another person was charged with obstruction of justice and conspiracy for attempting to conceal and destroy evidence, Ms. Larson said.

U.S. District Court records did not show any names of attorneys representing the group or scheduled court appearances.

Alvarado is a community of about 6,225 people about 40 miles southwest of Dallas.