WASHINGTON — Last November, officials in Lowndes County, Ala., began fielding inquiries from an unexpected inquisitor — the Justice Department, which had opened an investigation into the link between environmental racism and the chronic water, flooding and sanitation woes in the area.

The Biden administration’s choice of Lowndes as the site of its first big environmental justice inquiry was based on the magnitude of the county’s problems. But it also sent a message. The county was a voting rights battleground and a focal point of Martin Luther King Jr.’s march from Selma to Montgomery in 1965, making it a logical choice to open a new front on civil rights.

Last week, Attorney General Merrick B. Garland announced a series of policies intended to elevate the department’s environmental justice efforts from the symbolic to the substantive — as President Biden’s allies questioned the pace of other White House efforts to help Black and Hispanic communities hit hard by pollution, neglect and climate change.

Mr. Garland’s most important step, by far, was the creation of an office inside the department responsible for addressing the “harm caused by environmental crime, pollution and climate change” in communities of color and in low-income cities, towns and counties.

The new office is tiny — a $1.4 million operation tucked into a $35 billion department — but its ambitions are vast, and very much a work in progress. Its small staff of attorneys, led by a veteran environmental lawyer, Cynthia Ferguson, will have sweeping power under Title VI of the Civil Rights Act of 1964 to initiate investigations, negotiate civil settlements and pursue criminal indictments in the most extreme cases.

“It was a long time coming — we asked them more than 10 years ago to create a unit that would marry civil rights with environmental enforcement,” said Vernice Miller-Travis, a Maryland-based environmental justice activist who began pushing for the creation of a special office under the Obama administration.

“There are a lot of questions about how it will operate and it doesn’t have a big budget, but it represents a major commitment and it will grow,” Ms. Miller-Travis added.

Justice Department officials would not say what investigations the office will pursue first, which environmental issues it will tackle or when they plan to release their findings in their pathfinder Lowndes County case, a critical document that could offer a blueprint for how far the administration is willing to go.

But several people familiar with the process, speaking on the condition of anonymity to publicly discuss the matter, said potential targets for investigators were obvious to anyone who had followed the issue in recent years. They include a dense area of industrial refineries and plants in Louisiana known as Cancer Alley, parts of the Texas coast fouled by petrochemicals and majority-minority communities that have been relegated to low-lying, flood-prone areas without functioning water management infrastructure, like Cairo, Ill., and Sand Branch, Texas.

The department officials overseeing the effort — Associate Attorney General Vanita Gupta and Kristen Clarke, the head of the civil rights division — emphasized that no decisions had been made yet, and that investigations would be determined by local leaders who had firsthand knowledge of problems in their communities.

Moreover, every U.S. attorney has been instructed to dedicate lawyers to pursue environmental justice cases, and those prosecutors will have wide latitude to open their investigations, a force multiplier that should expand the effort far beyond department headquarters.

“That’s why it’s so important that these initiatives not only create a new office here in Washington, but also direct all 93 U.S. attorney’s offices across the country to proactively seek out information,” Ms. Gupta said in an interview.

Mr. Biden — who prevailed in the 2020 Democratic primaries by winning strong support from Black voters — was the first president to embrace environmental justice, the idea that all people have the right to protection from environmental and health hazards, as a core policy priority.

One of his first actions as president was to issue an executive order directing the Justice Department to “ensure comprehensive attention to environmental justice.” Enforcement of environmental regulations slackened during President Donald J. Trump’s four years in office, and his aides scrapped several key initiatives, including a program that required companies to bankroll community infrastructure investments instead of paying fines for violations of environmental law.

Still, Mr. Biden’s agenda has been hampered by delays and staffing turnover, prompting activists to worry that the administration could fall short of its promise to make environmental justice a priority across all parts of the federal government.

Last week, the White House Council on Environmental Quality tapped Jalonne L. White-Newsome, a longtime advocate for environmental equity, to be its senior director for environmental justice. She succeeds Dr. Cecilia Martinez, who, citing burnout, resigned in January after about a year in the position.

She will help oversee the Biden administration’s completion of a sweeping screening tool that tries to identify communities bearing the brunt of environmental damages.

Its goal is to help fulfill another presidential promise: to deliver 40 percent of the benefits of federal investments in environmental cleanup, clean energy and climate mitigation in disadvantaged communities.

The creation of the system faced months of delays, frustrating advocates and White House advisers who saw it as critical to directing money from the $1.2 trillion bipartisan infrastructure law that passed last year to struggling communities.

Jane English, program manager for the N.A.A.C.P.’s environmental and climate justice program, said that vow had yet to be fulfilled, and making good on it would require sustained monitoring of funding decisions.

She praised the Justice Department’s new office as “moving in the right direction” and said its creation was long overdue.

Not everyone was so supportive. Conservatives seized on the announcement as proof that Democrats were more interested in using federal law enforcement to pursue liberal political goals than addressing the national increase in violent crime, or countering threats to members of the Supreme Court.

“Instead of protecting the entire foundation of the Justice system, AG Garland set up a new Environmental Justice Unit this week,” Adam Paul Laxalt, a Republican and a former Nevada attorney general who is running for the Senate, wrote on Twitter. “Our failed ruling class in a nutshell.”

But administration officials say the Justice Department is more than capable of juggling multiple issues simultaneously.

The E.P.A. also has the power to investigate violations of federal environmental laws, and has stepped up its enforcement actions in communities of color under its administrator, Michael Regan, the first Black man to run the agency.

Over the past year, the E.P.A. has reached a $118 million settlement with Chevron Phillips to resolve Clean Air Act violations in Texas, halted operations at a refinery in the U.S. Virgin Islands and ordered officials in Clarksburg, W.Va., to provide an alternative source of drinking water or remove lead for customers affected by high levels of the contaminant, many of them Black.

The E.P.A., as part of Mr. Biden’s new push, will revive a program that allowed companies to reduce the penalty on some violations in exchange for subsidizing initiatives that have a positive environmental impact. The Trump administration had scrapped that program.

But the E.P.A. is fundamentally a regulatory and rule-making agency. It has addressed racial issues, but it lacks the legal and investigative firepower of Mr. Garland’s department.

The Justice Department has a robust civil rights division responsible for enforcing the 1964 Civil Rights Act, including Title VI, which prohibits states, localities and universities that receive federal funding from discriminating against people “on the basis of race, color or national origin.”

Activists began pressuring the Justice Department to use Title VI on environmental cases during the Obama administration. But that effort stalled, and the Trump administration quickly jettisoned the policy.

That all changed in November, when Ms. Clarke announced, to the surprise of local officials, that she was opening a Title VI investigation into the actions of the Alabama Public Health Department and the Lowndes County Health Department.

The state government, controlled by Republicans, has done little to address the problem on its own over the years. But local residents have also complained about the actions of local Black officials who have failed to address a sanitation and water management system that routinely results in sewage backups in the streets of towns like Hayneville.

Catherine Coleman Flowers, a MacArthur fellow whose 2020 book, “Waste,” brings attention to the crisis in Alabama, praised the creation of the office. But she said the ultimate test would be whether Mr. Garland was willing to withstand the political blowback for getting tough on governments, especially in conservative-leaning states.

“Environmental justice has to be done like any other investigation the department does,” like a corruption case, said Ms. Flowers, who has been interviewed by investigators. “They can do settlements, but people need to be punished when they do something wrong. There needs to be real penalties, or there will be no fear of repercussions.”