At least 52 members of an Iranian opposition group exiled in Iraq were executed over the weekend, the United Nations said Tuesday, drawing loud condemnations from supporters in Washington, who have called on the United States to find a safe haven for the group’s members.

The attack Sunday on a camp north of Baghdad was the latest targeting members of the Mujahideen-e Khalq, a group once aligned with Saddam Hussein, whose members disarmed following the 2003 U.S.-led invasion of Iraq after getting assurances from the U.S. military that they would be protected. Their fate since the U.S. military withdrawal in 2011 has been one of the most dramatic and violent subplots of the post-American era in Iraq.

Iraq’s Shiite-led government, which has long loathed the group, has been widely suspected of involvement or acquiescence in a string of attacks against the group — also known as the MEK — for which no one has been held responsible. Sunday’s was particularly brutal, according to photos, which showed the corpses of several men shot in the head and back, some with their hands bound.

Gyorgy Busztin, a U.N. envoy in Baghdad, visited the camp in Diyala province a day after the killings and spoke to survivors. In a statement, he expressed “his outrage at the brutal killing of the camp’s residents” and called on the Iraqi government to launch an investigation and “acknowledge its responsibility for the safety of the camp’s residents.”

Iraqi Prime Minister Nouri al-Maliki ordered an investigation into the killings, according to news reports in Baghdad.

Former Pennsylvania governor Ed Rendell, who is among the prominent Americans who champion the cause of the MEK, said he was horrified that the United Nations and the United States haven’t done more to get the Iranians out of Iraq.

“We are a joke when it comes to living up to our moral obligations,” Rendell said. “It is our legal and moral obligation to protect them.”

Rendell and the other prominent American MEK supporters have received speaking fees from the group, but the former governor said they were making the latest lobbying effort pro bono.

The status and fate of the MEK has long been a thorny issue for U.S. officials in Baghdad. After the invasion, most members lived in Camp Ashraf, a sprawling compound where the Iranians grew their own food, plotted to overthrow the Iranian government and lived according to strict behavioral edicts, including abstaining from sex.

Because of its militant past, the MEK was labeled a terrorist organization by the United States until last year.

The government in Baghdad sought for years to expel the group from Camp Ashraf, but that goal became possible only after the U.S. military pulled out of Iraq at the end of 2011. Most members of the group were transferred to a former U.S. military base in Baghdad called Camp Liberty, where they have come under attack a handful of times.

The members killed over the weekend were among the 100 or so who remained in Diyala province.

The State Department on Sunday called “on Iraqi authorities to act with urgency to immediately ensure medical assistance to the wounded and to secure the camp against any further violence or harm to the residents.”