(CNN) A group of 18 senators requested additional information about overcrowding at a border facility in a letter to acting Department of Homeland Security Secretary Kevin McAleenan Wednesday.

The letter stems from a May DHS inspector general report that found "dangerous overcrowding" and unsanitary conditions at an El Paso, Texas, Border Patrol processing facility following an unannounced inspection. The report's release in late May sparked outrage from lawmakers and illustrated the issues facing the Department of Homeland Security, primarily the challenge it faces in processing a growing number of migrants crossing the border.

"This is unacceptable and requires immediate action by DHS leadership to ensure that such situations never occur again," the senators -- led by Nevada's Democratic Sen. Jacky Rosen -- wrote in the letter, citing cells which held migrants over the maximum capacity.

The IG found "standing room only conditions" at the El Paso Del Norte Processing Center, which has a maximum capacity of 125 migrants. On May 7 and 8, logs indicated that there were "approximately 750 and 900 detainees, respectively."

"We also observed detainees standing on toilets in the cells to make room and gain breathing space, thus limiting access to the toilets," the report, which was first obtained by CNN, stated.

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