The Supreme Court on Wednesday said law enforcement officials had flexibility to enter a home without a warrant based on reports that someone inside might need emergency help, a decision with implications for police tactics and the expectation of privacy in one’s home.

In a unanimous decision, the justices said that the police in Montana had acted appropriately when they entered an Army veteran’s home without a warrant because they had an “‘objectively reasonable basis for believing’ that a homeowner intended to take his own life and, indeed, may already have shot himself,” Justice Elena Kagan wrote for the court.

The Fourth Amendment of the Constitution prohibits unreasonable searches and provides protections for a person’s home by generally prohibiting law enforcement from entering without a warrant.

The Supreme Court has carved out several exceptions, including for when police believe an occupant is seriously injured or facing an imminent threat of injury.

The question in the Montana case was what level of certainty police must have that an emergency is underway before they can enter a home without a warrant.

Police were called to the home of William Trevor Case in September 2021 by his ex-girlfriend, who feared he was suicidal. The Army veteran had a loaded handgun, she told police, and he had previously threatened to kill himself.

Mr. Case was well known to law enforcement officers who went to check on him at his home near Butte, Mont. Mr. Case had “tried this suicide by cop” stuff before, one of the officers said, using profanity, according to a body-cam recording of the police response.

The officers knocked on Mr. Case’s door, yelled and shined flashlights through the windows. They could see empty beer cans, an empty handgun holster and a notepad with handwriting, which the officers thought was a possible suicide note, court records show. After about 40 minutes, they entered through the unlocked front door without a warrant.

When Mr. Case suddenly emerged from a closet, he stretched out his arm with what appeared to be a gun, and an officer shot him in the abdomen. The veteran, who survived, was convicted of assaulting the officer.

He appealed that conviction, arguing that a gun and other evidence from his home should not have been allowed to be presented at trial because the officers had violated the Fourth Amendment by coming into his home without a warrant.

Mr. Case’s lawyer told the court that police should have met a high bar of “probable cause” for the intrusion — a standard his lawyer said would provide “a level of certainty that avoids needless and dangerous confrontations, and enables police and emergency medical workers to provide aid when occupants urgently need it.”

But the court on Wednesday declined to adopt that higher standard, which would have been borrowed from the criminal context. Instead, it reaffirmed a 2006 decision: that it is not a violation of the Fourth Amendment when the police make a warrantless entry, if officers have an “objectively reasonable” basis to believe that an occupant is “seriously injured or threatened with such injury.”

The court said it was reasonable for the police to believe Mr. Case needed emergency aid, based on the phone call with Mr. Case’s ex-girlfriend and what the officers could observe at the Army veteran’s home.

“If Case had already shot himself, he could have been severely injured and in need of immediate medical care. And if he had not, the risk of suicide remained acute, given all the facts then known to the officers,” Justice Kagan wrote.

Justices Sonia Sotomayor and Neil M. Gorsuch joined the majority but wrote separately, with Justice Sotomayor cautioning that it may not always be “objectively reasonable” for police responding to a mental health crisis to make a warrantless entry.

She cited studies showing that people with serious mental health conditions were disproportionately likely to be injured and killed during police interactions compared to the general population. The justice also warned that the presence of law enforcement could escalate such situations, “putting both the occupant and the officers in danger.”

The “objectively reasonable basis” test affirmed by the court, Justice Sotomayor wrote, “demands careful attention to the case-specific risks that attend mental health crises, and requires officers to act reasonably in response.”

In response to the ruling, the Constitutional Accountability Center, which had filed a brief in support of Mr. Case’s position, expressed concern that the court’s decision would do nothing to prevent officers from using emergency aid as a pretext for home intrusions.

Austin Knudsen, Montana’s attorney general, praised the court’s ruling, saying in a statement that it would give law enforcement the ability to respond to life-threatening emergencies and “allow officers to continue to keep their communities and citizens safe to the best of their abilities.”