Relatives hold a banner reading "Disappeared" as they march along Reforma Avenue in Mexico City on May 10 to demand justice for their kin who have been forcibly disappeared. (Henry Romero/Reuters)

The United Nations has “strong indications” that Mexican security forces were involved in the disappearances of 23 people over the past four months who were plucked off the streets of a Mexican city along the U.S. border, a top U.N. official said Wednesday.

The U.N. investigation has documented the disappearances of 21 men and two women, including minors as young as 14, in Nuevo Laredo between February and May 16. Local human rights groups have reported that even more people have gone missing in that period from the border city. Some relatives of the victims blame Mexico’s navy for the disappearances.

“Many of these people are reported to have been arbitrarily detained and disappeared while going about their daily lives,” the U.N. high commissioner for human rights, Zeid Ra’ad al-Hussein, said in a statement. “These crimes, perpetrated over four months in a single municipality, are outrageous.”

The United Nations reported that the Mexican government has made little progress locating the disappeared, “despite ample information and evidence,” and added that “several witnesses have been subjected to threats.” According to testimony gathered by U.N. investigators, federal security forces had picked people up late at night or early in the morning, “as they walked or drove along public roads.” The U.N. statement did not identify the security forces.

Families searching for relatives have found six bodies.

President Enrique Peña Nieto’s office referred queries about the U.N. statement to the Foreign Ministry, which did provide an immediate response. The navy did not respond to a request for comment.

Mexico has endured record levels of violence in the past year as the drug war rages in many parts of the country, including in the states along the U.S. border. Mexican production of opium poppy and heroin has risen to meet American demand, and the quantity of methamphetamines and other drugs flowing into the United States has also increased.

Mexico’s military has a visible presence in many hot spots, and the navy’s elite marine units have played a key role in anti-drug operations. The military has faced regular accusations of human rights violations, including torture, disappearances and extrajudicial killings. A report last year by the Washington Office on Latin America (WOLA), a research and advocacy group, found that about 97 percent of human rights violations committed by Mexican soldiers go unpunished.

Concern about disappearances in Nuevo Laredo, a city across the Rio Grande from Laredo, Tex., has been building in recent months. Relatives of the disappeared blocked the border bridge this month, demanding that the navy leave their city and that the Mexican government investigate more than 40 cases of people who had been abducted.

“The silence of the government in these situations is really unacceptable,” said Ximena Suarez-Enriquez, assistant director for Mexico at WOLA. It is “necessary now for the government to come out and clarify if they are investigating these cases and if there are members of the navy or the military involved.”

In the state of Tamaulipas, which includes Nuevo Laredo, the threat of violence and the power of drug cartels often make it difficult for human rights groups and journalists to investigate such allegations. A civil society group called the Network of Disappeared of Tamaulipas has a database of more than 1,300 unresolved disappearances in Nuevo Laredo since 2006, said its president, Josefina de Leon.

She added that sometimes organized crime groups wear police and military uniforms, making it even more difficult to identify those responsible.

“The state has little capacity to find the disappeared” or prevent the phenomenon from happening, she said.

Last month, Mexico’s National Human Rights Commission sent a message to the navy and other security bodies urging them to protect civilians in Tamaulipas.

Mexico recently passed a law intended to improve the government’s ability to track and investigate disappearances. Zeid called it “extremely worrying that these enforced disappearances are taking place just a few months after the adoption” of the new law.

Gabriela Martinez contributed to this report.