The arrest of a Vietnamese green energy expert who had worked with U.N. and U.S. agencies, just days after President Joe Biden inked business and human rights agreements with Hanoi on a visit, has raised concerns with the U.N. human rights office.

According to Reuters last week, who cited a charity and a source, Ngo Thi To Nhien, the executive director of the Vietnam Initiative for Energy Transition (VIET), an independent think tank specializing in green energy policy, was detained by Hanoi police on September 15.

“We are aware of the arrest and are following the developments with concern,” Ravina Shamdasani, a spokesperson for the UN Office of the High Commissioner for Human Rights (OHCHR) told Reuters in a statement.

Nhien had worked for the World Bank, with the United Nations Development Programme and the United States aid agency (USAID), according to her profile on LinkedIn.

She “has participated in international and national events, including consultations organized by UNDP on the topic of energy transition,” the UNDP in Vietnam confirmed in an email message to Reuters.

A State Department official said Washington regularly calls on Vietnam to respect and protect human rights, but had no specific comment on Nhien’s detention and its timing close to Biden’s visit.

Over the last two years Vietnam has arrested five environmental human rights defenders accusing them of tax evasion, a OHCHR spokesperson said in June, noting the arrests happened while the country was negotiating international funding for energy transition away from coal, of which it is a major user.

Nhien kept a very low public profile and was considered an expert, not an activist.

Vietnam’s government has not issued any public statement about Nhien’s arrest, and did not reply to requests for comment.

On Friday, Vietnam also executed a man, Le Van Manh, who had been sentenced to death in July 2005 after being found guilty of murder, child rape and robbery.

The European Union had called to halt the execution.

Phil Robertson, Deputy Asia Director at Human Rights Watch, said Manh had a strong alibi which was disregarded.