The Justice Department on Thursday announced a sweeping civil rights investigation into the Phoenix Police Department, marking the third such inquiry that the Biden administration has opened into abuse allegations against a major national police force.

The investigation will examine whether the Phoenix police discriminate against minorities, use excessive force or retaliate against peaceful protesters. The inquiry will also scrutinize the department’s treatment of homeless people and disabled people, Attorney General Merrick B. Garland said in announcing the investigation at a news conference in Washington, emphasizing that police officers are often called upon to handle mental health emergencies and other social issues.

“Our society is straining the policing profession by turning to law enforcement to address a wide array of social problems,” Mr. Garland said. “Too often, we ask law enforcement officers to be the first and last option for addressing issues that should not be handled by our criminal justice system. This makes police officers’ jobs more difficult, increases unnecessary confrontations with law enforcement and hinders public safety.”

The decision to open the investigation was based on a review of court documents and public reports, said Mr. Garland and Kristen Clarke, the assistant attorney general for civil rights. They did not cite a specific event that prompted the inquiry.

The Phoenix Police Department, which is among the 10 largest in the country, has a history of mistreatment of minorities and disabled people. One of the department’s officers admitted to pushing a blind man to the ground in a convenience store bathroom in 2018 after the officer said the man came too close to him. A scuffle ensued, and the police arrested the blind man and charged him with a felony. The department also had a spate of officer-involved shootings in 2018.

The investigation is part of the Biden administration’s emphasis on oversight of the country’s police departments, which have come under immense scrutiny in recent years. Mr. Garland has also announced inquiries into the Minneapolis and Louisville police departments after the high-profile killings last year of George Floyd and Breonna Taylor.

Such investigations often lead to court-approved agreements between the Justice Department and local governments that are intended to enhance police training and oversight.

Phoenix’s mayor, Kate Gallego, said she welcomed the investigation. “The recommendations that will result from this review will assist us in our ongoing efforts to become an even safer, stronger, more equitable city,” she said in a statement.

Activists in Phoenix who for years have denounced police violence said they saw the investigation as a positive step, even as they warned against solutions that often arise from such inquiries, such as more money for training.

“We had a sigh of relief in some ways because the violence we have been exposing for years has finally been seen at a national level,” said Viri Hernandez, the executive director of Poder in Action, a nonprofit that works with people who have lost loved ones to police violence.

After the murder of Mr. Floyd while in police custody last year in Minneapolis, activists around the country, including in Phoenix, demanded that police departments be defunded. While the movement has gained traction in some cities, like Minneapolis and Los Angeles, Ms. Hernandez said officials in Phoenix had shown little appetite for cutting the police budget.

“For years we have been saying this, and last year after George Floyd our demands were echoed in the streets,” she said, quickly adding, “Defunding is not happening here even though community members have made that a priority.”

In recent years, both Arizona and Maricopa County, which includes Phoenix, have had some of the highest rates of police shootings that resulted in deaths. Analyzing data from 2015 to 2020, one study found that Arizona had the fourth-highest rate of police killings among states, while Maricopa County had the ninth-highest rate among the nation’s 100 most populous counties.

At the same time, homelessness has become a growing concern in Phoenix, along with other cities, especially in the western United States, where more people are living on the streets amid a pandemic and mental health and addiction crises. Alarming activists and those experiencing homelessness has been the police response: a steady flow of early-morning sweeps of homeless encampments, in which officers seize people’s belongings.

“Police are just playing Whac-a-Mole, just pushing them from place to place,” said Elizabeth Venable, the founder of the Fund for Empowerment’s Houseless Leadership Project, which advocates on behalf of the homeless.

Ms. Venable said she was contacted this week by officials at the Justice Department to participate in the investigation, and she said she would organize groups of people to meet with federal investigators to discuss the police response to homelessness.

Police representatives pledged to work with the Justice Department. The president of the police union in Phoenix, Michael London, said that its priority was to maintain high policing standards. “We are confident in the work of the Phoenix Police Department and our officers on the street, and we will fully cooperate with the Department of Justice investigation,” he said in a statement.

The Trump Justice Department had largely curbed the use of court-approved agreements and shied away from opening civil rights investigations into police departments. Senate Republicans had also sought unsuccessfully to block the nomination of Ms. Clarke, a civil rights lawyer who has vowed to use the civil rights division to take on systemic racism, hate crimes and restrictive voter laws.

“One of the highest priorities of the civil rights division is to ensure that every person in this country benefits from policing that is lawful, effective, transparent and free from discrimination,” Ms. Clarke said on Thursday. “Police officers across the country must use their authority in a manner that adheres to the Constitution, complies with federal civil rights laws and respects human dignity.”