By Sara Ganim, CNN

(CNN) -- House Democrats are asking Monday to subpoena records that they think will show that Environmental Protection Agency Administrator Scott Pruitt used his staff to run personal errands, including trying to buy a used mattress from the Trump International Hotel in Washington during his controversial apartment search.

In an interview with House Oversight Committee members, one of Pruitt's top staffers who describes herself as a personal friend of Pruitt, Millan Hupp, told investigators that she searched for rental properties for Pruitt and his wife for several hours a week over the course of several months, touring more than 10 of those properties during work hours.

Hupp, who works as a scheduler at EPA, said it was mostly on her lunch break, which she felt was appropriate, but Democrats on the committee are alleging it's a violation of rules that prevent federal employees using public office for private gain. They're asking Oversight Committee Chairman Trey Gowdy, a South Carolina Republican, to issue subpoenas for communications related to her work.

When asked about the letter, EPA spokesman Jahan Wilcox said, "We are working diligently with Chairman Gowdy and are in full cooperation in providing the Committee with the necessary documents, travel vouchers, receipts and witnesses to his inquiries."

A committee spokeswoman said the panel "has conducted several transcribed interviews and obtained 2,350 pages of documents as part of our investigation into mismanagement and spending at the EPA," adding, "This month, the Committee will interview additional witnesses and receive additional document productions.

"Selectively releasing portions of witness interview transcripts damages the credibility of our investigation and discourages future witnesses from coming forward," Oversight Committee spokeswoman Amanda Gonzalez said in a statement. "The Committee will continue conducting a serious, fact-driven investigation, and therefore will wait until the conclusion of our investigation to release our findings."

In addition, Hupp reached out to the managing director of the Trump International Hotel in Washington, after Pruitt "indicated that there could be a mattress that he could purchase," Hupp told the committee in her May 18 interview.

Hupp couldn't recall if the mattress purchase went through, but said it was around the time of Pruitt's apartment search.

"As I remember, the administrator had spoken with someone at the Trump Hotel who had indicated that there could be a mattress that he could purchase, an old mattress that he could purchase," Hupp said according to a transcript the interview. "But that's the extent of the conversation that I can -- that I can remember."

Hupp also told the committee that she used Pruitt's personal credit card to purchase plane tickets for his family vacation to the Rose Bowl and Disneyland over the winter holiday, but said she did that on her own time during her holiday vacation.

"He just sent me the flight details and asked me to book for him," she told the committee, adding, "I used my work email to share details with the security team."

That trip has come under scrutiny since Pruitt used his 24/7 security team to accompany him at an unknown cost to the EPA.

Hupp is one of several employees in Pruitt's inner circle who came with him from Oklahoma and is one of the women who received the controversial pay raise that Pruitt had to rescind after denying he knew the details.

Hupp had been hired with a $66,500 salary, which skyrocketed to $114,600 before it was rescinded.

Hupp said she was not paid for her non-EPA errands for Pruitt.

Pruitt has previously defended Hupp's involvement in the apartment search, citing friendship and the unpaid nature of her work, but ethics experts have raised issues with her doing the search, whether she was on the government clock, volunteering for Pruitt, or paid separately by Pruitt.

"It's my understanding that all activity there was on personal time," Pruitt said at a mid-May Senate hearing. "And the individual that you're referring to is a longtime friend of my wife and myself."