U.S. Border Patrol agents who detained a 7-year-old Guatemalan migrant girl who later died in federal custody will not speak with U.S. lawmakers investigating her death, the U.S. Department of Homeland Security said on Monday, citing their union membership.

Members of the Congressional Hispanic Caucus on Tuesday plan to visit the New Mexico patrol station where Jakelin Caal and her father were taken on Dec. 7 to learn more about why she died the next day.

Caal’s death fueled criticism of President Donald Trump’s immigration policies from Democrats and migrant advocates.

Department of Homeland Security (DHS) Secretary Kirstjen Nielsen said it showed the dangers of her journey and the family’s decision to cross the border illegally.

U.S. Representative Joaquin Castro, a Democrat who leads the Hispanic Caucus, said lawmakers needed to see how Border Patrol stations were equipped to protect the health and safety of migrants. The lawmakers want to question agents who were present when Caal became ill.

AP Photo/Oliver de Ros Claudia Maquin, 27, walks with her three children, Abdel, 9, left, Angela, 6 months, middle, and Elvis, 5, right, as they leave her father-in-law's house, in Raxruha, Guatemala, on Dec. 15, 2018. Her daughter, 7-year-old Jakelin Caal, died in a Texas hospital after being taken into custody by border patrol agents in New Mexico.

DHS said the majority of agents involved in the girl’s detention were union members and, as such, their duties did not include speaking to members of Congress.

Art Del Cueto, a spokesman for the agents’ union, the National Border Patrol Council, said it advised them not to speak to anyone outside DHS about the incident until the agency completes its investigation.

The DHS said visiting lawmakers would be able to speak with the commissioner of U.S. Customs and Border Protection (CBP) and Border Patrol leaders.

“While DHS respects the oversight role of Congress, it is important to allow the review process to occur unimpeded by politics,” DHS spokeswoman Katie Waldman said in a statement.

Agents involved in the incident will participate in the DHS inspector general’s investigation of Caal’s death, DHS said.

The Congressional Hispanic Caucus did not immediately respond to a request for comment.

Caal and her father crossed a remote stretch of border in a group of 163 immigrants and were apprehended by Border Patrol. They waited eight hours to be transported to the nearest Border Patrol station. On arrival at the Lordsburg, New Mexico station, Caal was found to have a high fever. She died Dec. 8 in an El Paso, Texas hospital after being diagnosed with liver failure, according to CBP.

Attorneys for the family released a statement on Saturday calling for “a transparent and neutral” investigation into her death. The family disputed a statement made by CBP officials to the Washington Post that Jakelin had not eaten or had water in several days.

(Reporting by Andrew Hay in New Mexico; Editing by Bill Tarrant

and Lisa Shumaker)