An investigation by a nonprofit housing advocacy group found that D.C. landlords or rental agents refused or imposed illegal conditions on 45 percent of renters wanting to use federal housing vouchers.

The Equal Rights Center, which had conducted a similar investigation five years ago, said Thursday that in tests at 91 rental properties, landlords refused to rent to 15 percent of applicants who offered the vouchers as payment and tried to impose illegal conditions on an additional 30 percent who presented the vouchers.

“One landlord imposed a requirement that a voucher holder had to make $64,000 a year in order to a rent an apartment costing $1,300 a month,” said Don Kahl, the center’s executive director. Applicants for the voucher program typically have incomes of less than $30,000 for a family of four.

Such actions are illegal under the D.C. Human Rights Act, which prohibits landlords from discriminating because a renter wants to pay using various types of funds, including government vouchers.

The Equal Rights Center acts as an advocate on behalf of people who face discrimination and will take a complaint against a landlord to the D.C. Office of Human Rights. But people seeking housing are often reluctant to make complaints. The vouchers are good for only 90 days, with up to two 30-day extensions available. Those who don’t find housing in that period lose the voucher and have to rejoin a waiting list of 34,717 families to get it again. About 11,000 families in the District use the vouchers.

Gustavo F. Velasquez, director of the Human Rights Office, said complaints of housing discrimination based on a person’s income have gone up in the past few years.

“By far the most common complaint is discrimination based on disability,” Velasquez said.

Five years ago, a similar investigation by the center found that landlords refused or tried to limit the use of vouchers by 61 percent of applicants; since then, more than 20 agreements between the center and property owners or managers opened more than 15,000 apartments to voucher holders.

“While there has been improvement, voucher holders still face major barriers in finding decent housing,” Kahl said.

In the past five years, the Equal Rights Center and the Human Rights Office have trained housing seekers and landlords and property managers, attempting to educate everyone about their rights and obligations. More than 20 enforcement actions were launched, although none has resulted in a fine, Velasquez said. A potential tenant may also sue in civil court.

The Equal Rights Center’s investigation involved trained civil rights testers posing as housing voucher users who contacted a property owner or manager to be sure an apartment was still available and priced as advertised.

The properties were in all four quadrants of the city and ranged in size from efficiencies to two-bedroom units. The choice of who would be contacted was not a scientifically random sample, and there was no indication whether the landlords who were reached were versed in the anti-discrimination housing law.

In 14 of 91 tests, landlords refused to rent to people offering vouchers. One housing provider immediately hung up the phone when a tester asked whether a voucher could be used to pay the rent; another said he “did not believe in vouchers.” In other instances, a landlord claimed his property had reached its “quota” of voucher holders, and another wrongly said that vouchers provided by the renter had to cover the entire cost of the rent.