The Supreme Court building in Washington, D.C. 

WASHINGTON — The Supreme Court on Monday ruled for an Arizona police officer who shot a woman outside her home in Tucson. The court’s decision was unsigned and issued without full briefing and oral argument, an indication that the majority found the case to be easy.

In an impassioned dissent, Justice Sonia Sotomayor said the majority had gone badly astray.

“Its decision is not just wrong on the law; it also sends an alarming signal to law enforcement officers and the public,” she wrote. “It tells officers that they can shoot first and think later, and it tells the public that palpably unreasonable conduct will go unpunished.”

The case started in 2010 when three police officers responded to a 911 call reporting that a woman had been seen acting erratically by hacking at a tree with a knife.

They saw Sharon Chadwick standing in the driveway of a house. A second woman, Amy Hughes, emerged from the house, holding a kitchen knife. She stopped six feet from Ms. Chadwick.

Although the officers did not know it, the two women were roommates. Ms. Hughes was not moving, spoke calmly, held the knife at her side and made no aggressive movements.

Ms. Chadwick later said that she did not feel threatened and that Ms. Hughes had appeared composed.

The officers drew their guns and told Ms. Hughes to drop the knife, but it is not clear that she heard them. Officer Andrew Kisela opened fire, shooting Ms. Hughes four times.

Screaming and bleeding, Ms. Hughes asked, “Why’d you shoot me?”

She survived and sued the officer for using excessive force. The United States Court of Appeals for the Ninth Circuit, in San Francisco, allowed the case to proceed.

The Supreme Court reversed that ruling, saying that Officer Kisela was entitled to qualified immunity, a doctrine that shields officials from suits over violations of constitutional rights that were not clearly established at the time of the conduct in question.

The majority did not decide whether Officer Kisela’s actions violated the Constitution, but it did say there was no clear precedent that would have alerted him that opening fire in what he said was an effort to protect Ms. Chadwick amounted to unconstitutionally excessive force.

In dissent, Justice Sotomayor, joined by Justice Ruth Bader Ginsburg, said the majority’s reasoning was perplexing.

“Hughes was nowhere near the officers, had committed no illegal act, was suspected of no crime, and did not raise the knife in the direction of Chadwick or anyone else,” Justice Sotomayor wrote, adding that only one officer had opened fire.

“Kisela alone resorted to deadly force in this case,” she wrote. “Confronted with the same circumstances as Kisela, neither of his fellow officers took that drastic measure.”

Justice Sotomayor said a jury should have been allowed to decide the case.

“Because Kisela plainly lacked any legitimate interest justifying the use of deadly force against a woman who posed no objective threat of harm to officers or others, had committed no crime, and appeared calm and collected during the police encounter,” Justice Sotomayor wrote, “he was not entitled to qualified immunity.”

Justice Sotomayor said the court’s decision in the case, Kisela v. Hughes, No. 17-467, was part of a disturbing trend of “unflinching willingness” to protect police officers accused of using excessive force.

The court’s decisions concerning qualified immunity, she wrote, “transforms the doctrine into an absolute shield for law enforcement officers.”

“Because there is nothing right or just under the law about this,” she wrote, “I respectfully dissent.”