Hanoi, VIETNAM (AP) — Vietnam has detained the director of a think tank that works on energy issues in the country — the sixth expert working on environmental and climate issues that authorities have taken into custody in the past two years, a rights group said Wednesday.

Ngo Thi To Nhien, the executive director for Vietnam Initiative for Energy Transition (VIET) was arrested on Sept. 15, according to The 88 Project, a group that advocates for freedom of expression in Vietnam.

Police also raided and searched the offices of the think tank and interrogated staff members, it said.

It was unclear why Nhien was arrested. Police have said the earlier arrests of other energy experts were on suspicion of tax evasion.

A person familiar with the situation who asked not to be further identified out of concern for their own safety confirmed that she had been detained. Police could not be reached for comment after business hours.

“Nhien’s detention is significant as it signals that research on energy policy is now off limits”, said Ben Swanton, of The 88 Project.

When she was arrested, Nhien was working with the United Nations Development Program to help implement the Just Energy Transition Partnership, or JETP — a deal designed to help the Southeast Asian nation phase out use of fossil fuels with $15.5 billion in support from the Group of Seven advanced economies, the advocacy group said.

The German government said in June that it was concerned by the earlier detention of a prominent environmental campaigner in Vietnam, warning that the JETP deal requires the involvement of civil society activists.

Nhien has worked in the past with other international organizations like the World Bank, the European Commission and the United Nations.

Vietnam is one of a few remaining communist single-party states that tolerate no dissent.

In 2022, Human Rights Watch said that more than 170 activists had been put under house arrest, blocked from traveling or in some cases assaulted by agents of the Vietnamese government in a little-noticed campaign to silence its critics.