The Department of Homeland Security has informed members of Congress that they cannot interview the Border Patrol agents who detained and processed a 7-year-old Guatemalan girl who died in their custody.

Members of the Congressional Hispanic Caucus on Tuesday are scheduled to tour the Lordsburg Border Patrol Station in New Mexico, where agents took Jakelin Caal Maquin and her father after they illegally crossed the U.S.-Mexico border. The members had requested a chance to speak directly with the Border Patrol agents who arrested, detained and transported the girl and her father before she was flown to a children's hospital and died.

A Homeland Security spokeswoman said Monday that the agents will already speak with the department's Inspector General, which is conducting an investigation into the girl's death, and that they can't be forced to speak with members of Congress while that process is ongoing.

"While DHS respects the oversight role of Congress, it is important to allow the review process occur unimpeded by politics," spokeswoman Katie Waldman said in a statement. "It is important to preserve the legitimacy of a fair and impartial process out of respect to the agents' lifesaving mission and the family of the deceased."

Maquin's death has reignited a debate over the way Customs and Border Protection handles migrants along the southern border. The agency came under fire over the summer after the Trump administration unveiled a "zero tolerance" policy that led to thousands of family separations. And it was scrutinized for firing tear gas into a group of asylum-seeking migrants in Tijuana, Mexico, that included men, women and children.

More:Migrant girl dies in custody: What we know now and where it happened

Related:Family of migrant girl who died in Border Patrol custody disputes agency’s account of death

In the case of the Guatemalan girl, CBP says its agents treated her as quickly and as effectively as they could after apprehending her and her father Dec. 6. The agency blamed the father for not alerting agents to her condition earlier. The father challenged that account, saying his daughter was fed and received water throughout their voyage and only became ill after being taken into CBP custody.

Now, Congress has many questions.

About a dozen members of the Congressional Hispanic Caucus, including several who represent districts in border states, will tour the Lordsburg station on Tuesday and speak with CBP leadership there.

Sen. Dianne Feinstein, D-Calif., the top-ranking Democrat on the Senate Judiciary Committee, sent a letter to CBP Commissioner Kevin McAleenan demanding a more detailed timeline and accounting of what happened to Maquin during her time in custody.

"Please also preserve any and all evidence related to the death," she wrote.

And on Thursday, the House Judiciary Committee will get its chance to ask questions when Homeland Security Secretary Kirstjen Nielsen appears before that committee.