WASHINGTON — House Democrats demanded documents from the Department of Homeland Security on Monday to investigate a “troubling pattern of abuse and poor treatment” of migrants, after the death of a 16-year-old Guatemalan detainee and other new revelations.

In letters sent to immigration and homeland security officials, the House Oversight and Reform Committee demanded to know how a teenage detainee, who had the flu and a 103-degree fever, could have been sent to a holding cell where he lay still next to a toilet for four hours, only to be discovered dead by his teenage cellmate.

Representative Carolyn B. Maloney of New York, the chairwoman of the House Oversight and Reform Committee, also questioned the shifting explanations from Customs and Border Protection, a division of the Department of Homeland Security. Officials initially said agents discovered the body of the teenager, Carlos Gregorio Hernandez Vasquez, during a welfare check. But surveillance video and additional documents, first obtained by ProPublica, revealed that to be untrue.

Customs and Border Protection officials also have yet to explain why they disregarded the recommendation of a nurse practitioner who asked that Mr. Hernandez Vasquez be re-evaluated and hospitalized if his condition worsened. Instead, he was left in his cell, where video showed him vomit before collapsing and lying motionless while his cellmate — also sick — slept under a foil blanket.