Mail is delivered after 5 p.m. more than two-thirds of the time to residents and businesses in the District and the Maryland suburbs, an investigation released Tuesday found. The late deliveries have raised concerns about the safety of letter carriers and provoked a deluge of complaints from customers.

The number of letter carriers who are still on their routes into the early evening — even in the winter darkness — has increased so dramatically that the Capital District now ranks as the worst region in the nation for late mail service, a report by U.S. Postal Service Inspector General David Williams concluded.

Northern Virginia also ranked in the top five areas of the United States receiving their mail late, auditors found, with 69 percent of letter carriers still on their routes at night in 2013.

Both regions were well above the national average for late deliveries from fiscal 2011 to 2013, with 38 percent of carriers out late last year. The delays violate the Postal Service’s goal of having 95 percent of city carriers return from street operations before 5 p.m. to maximize the efficiency of the long delivery-logistics chain. The yearly cost of overtime for carriers who are out late in the Capital District is $4.5 million, the inspector general previously found.

The inquiry was prompted by the November death of a part-time carrier who was shot in his mail truck in Prince George’s County at 7:30 p.m.

Authorities have made no arrest in the killing of Tyson Jerome Barnette, 26, who relieved full-time colleagues of excess workloads and was on a new, unfamiliar route the day he was killed.

The Postal Service told auditors it is working to change how the flow of work is structured and improving employees’ supervision and training. That includes reevaluating staffing levels, route structures and start times as well as better preparing for large volumes coming from sorting plants.

“Employee safety is a top priority at the U.S. Postal Service,” the agency said in a statement. “We have a robust safety program to heighten awareness of the range of situations carriers may face while performing their duties, and we teach techniques to minimize risks to them.”

The delays have resulted from a cascade of service cuts: The Postal Service, reeling from multibillion-dollar losses as Americans turn more to the Internet to correspond and do business, has shaved costs.

Officials have drastically reduced the size of the workforce, replacing thousands of letter carriers and other employees with part-time and contract employees. Hundreds of mail-sorting plants — including two in the Washington area — have closed.

The changes have led to disruptions in the mail’s finely calibrated journey from sender to home or business, resulting in mail from overburdened processing hubs arriving late at local post offices, with some of it not fully sorted, auditors found. Letter carriers often are left to finish the sorting by hand, delaying them from getting on their routes.

Supervisors, whose ranks have thinned with retirements, routinely fail to monitor and fix the problems, the report found, leaving a chaotic system that has angered customers who still rely on the mail for documents and correspondence.

Employees and their unions say they are concerned about their safety in high-crime neighborhoods after dark. “Each day some carriers deliver mail to locations that could be considered unsafe and deliveries late in the day can exacerbate potential dangers,” the inspector general wrote.

Postal officials announced plans this spring to accelerate closures of mail-sorting plants to save more money, which could result in continued delivery issues. In about the past 18 months, the Postal Service has quietly changed its service standards in many parts of the country to accommodate the closures, increasing the amount of time allowed for a letter to travel from one end of the country to the other by one day, sometimes two.

The Capital District includes Washington; Prince George’s, Charles, Calvert and St. Mary’s counties; and most of Montgomery and Anne Arundel counties. The number of carriers out after 5 p.m. rose 14 percent to nearly 70 percent from the first quarter of 2011 to the last quarter of 2013, auditors found. In that last quarter, 100,239 carriers on nearly 2,000 routes were delayed in their deliveries.

The other regions that showed poor performance were Richmond, Atlanta and South Florida, the report found.

Americans are complaining about chronic problems with late and missing mail. They sent the Postal Service 122,138 complaints in the second quarter of 2014, the highest number since the last quarter of 2011, when 79,709 complaints came in, according to data provided to The Washington Post under the Freedom of Information Act. The complaints eased to 84,700 in the second quarter of this year.

In the Washington area, auditors found that supervisors didn’t plan properly for a flood of new pieces of mail and didn’t effectively ensure that it got to local post offices early enough in the morning for letter carriers to get out on their routes.

Postal officials say they are investing in technology that allows supervisors to communicate in real time with carriers. But “later-day deliveries are sometimes unavoidable due to inclement weather, traffic issues or other unplanned events, as well as seasonal fluctuations in mail volumes,” the agency said.

Customers have noticed, and many are frustrated.

Chris Powell of Arlington County says he and his wife depend on the mail to receive checks, bills and disability paperwork. He said hours matter for people who are on a limited income and don’t bank online, as they may be counting on a paycheck arriving one afternoon to quickly turn around and pay bills with the next day.

“A lot of people don’t do all that electronic stuff,” said Powell, 43. “Sometimes, my whole Saturday is gone because I’m waiting on a check.”

In Shaw, Lynne Smith said she has sometimes seen her mail carrier as late as 11:30 p.m. She worries that letters and packages will be stolen if she waits until morning to bring them inside.

“Someone could just walk by and take the mail,” said Smith, 58.

Prince George’s County police and the U.S. Postal Inspection Service, meanwhile, are still investigating Barnette’s death, and they have offered a $125,000 reward for tips leading to a break in the case.

On Monday, postal workers from across the country began meeting in Philadelphia for the National Association of Letter Carriers’ convention, where they have made safety a central theme of the gathering. Several carriers from the Washington area created yellow buttons and T-shirts that say “Safety First,” which they’ll wear at the event.

Many will be thinking of Barnette, said Tom Dodge, who works in Baltimore and is the East Coast organizer for Community and Postal Workers United. “We haven’t forgotten,” Dodge said. “We’re not going to let this go. Heck, no.”

Dodge said he and other carriers will continue to press the Postal Service to hire more carriers and change how mail is processed to keep carriers from having to deliver in the dark.

“We’ve been frustrated since day one, but there isn’t anything we can do about it except pray,” said the slain carrier’s grandmother, Katherine Strong. “I don’t understand why anyone would shoot him like that.”