(Bloomberg) — The collapse of Zillow Group (NASDAQ:) Inc.’s home-flipping operation is highlighting the close ties between tech-powered real estate companies and the Wall Street landlords hunting for rental houses.

Zillow has sold homes in recent weeks to prominent institutional landlords, including rental operations tied to Cerberus Capital Management, KKR & Co (NYSE:). Tricon Residential is owned by Blackstone (NYSE ) Inc.

The Seattle-based realty giant has begun to pursue larger deals in order to sell approximately 18,000 properties. The company agreed this week to sell 2,000 properties to Pretium, a New York investment firm that’s the second-largest U.S. single-family landlord, with more than 70,000 houses. 

Zillow isn’t alone. Wall Street landlords looking for homes in the pandemic-hit housing market are also tapping iBuying competitors Opendoor Technologies Inc. and Offerpad Solutions Inc. Bloomberg reports that the companies sold 20% or more of their houses to investors last year.

“The bigger that number gets, the worse I feel about it,” said Mike DelPrete, a real estate tech strategist and scholar-in-residence at the University of Colorado Boulder. “I get that there are business considerations. Zillow’s ability to flip thousands of homes at once makes it a profitable business. But this is more than business, this is housing.”

Learn more: Cerberus leads Wall Street Landlords to Find Hidden Homes to Purchase

Zillow recently circulated a list with 9,000 properties for potential buyers. But that number may fluctuate, according people who are familiar with sales efforts and asked to remain anonymous because of the nature of the matter. According to one person, there are some investors who will underwrite bids for the entire portfolio.

Pretium’s homes were likely sold to Zillow for close to $750million, according to Berenberg Capital Markets analysts. 

Representatives of Zillow and Pretium were unable to comment.

“We intend to sell our remaining inventory — which represents less than three-tenths of 1% of all U.S. homes sold this year — the same way we always have: By selling to buyers of all types including individuals, families, individual investors, institutional investors and nonprofits,” Zillow said in a statement.  

Opendoor founder Keith Rabois, Chief Executive Officer Eric Wu and other venture capitalists created iBuyer to simplify the often complicated process of selling a house. They used price-based algorithms to place bids for properties. Then they made repairs and placed them on the market.

Learn more: Zillow home-flipping bonds draw Wall Street deeper into housing

As they’ve matured, buying thousands of homes a month, the iBuyers have allowed Wall Street firms to buy properties before they hit the open market, letting landlords jump ahead of regular buyers waiting for inventory to be listed. 

Bloomberg analyzed tens of thousand of Attom Data Solutions’ property records in order to identify the homes of three of the biggest iBuyers that were sold to corporations or legal entities. The list included more than 5,800 properties. Most of these were owned by institutional landlords.

Opendoor, the largest iBuyer, sold roughly 3,400 homes to those buyers this year, accounting for more than a quarter of the company’s sales, according to the analysis. Offerpad’s 21% was attributed to Zillow at 19%. This does not include the Pretium transaction.

Opendoor has declined to comment. Offerpad typically sells 10% to 20% of its home to single-family landlords, though it “ebbs and flows based on buying trends,” a representative said in an email.

Mounting Scrutiny

As both iBuyers and investors seek growth, the number of houses they purchase is expected to rise. It’s also likely to attract greater scrutiny. Politicians on both sides of the aisle are eyeing Wall Street’s role in housing. IBuyers have been also under scrutiny. The virality of a TikTok clip by a Las Vegas Realtor in September that suggested the companies were manipulating prices for homes was due to TikTok.

Read more: Wall Street’s Favorite Suburban Housing Bet Is Getting Crowded

Tina Smith, Sherrod brown, and Jack Reed, Democratic senators sent Rich Barton a note this week that mentioned a Bloomberg News report from Nov. 1. They wanted to know how many properties his company had sold to Wall Street companies and what its future plans were for selling them. 

Zillow spokeswoman said that the company had been in constant communication with the committee, and was working on information regarding the winding down of its iBuying division. 

A representative for Pretium, which was mentioned in the letter, said in a statement that the company “provides a high-quality housing experience through consistent, dependable and attentive service at our well-maintained and affordable homes.”

These questions are urgently needed as many first-time home buyers have been pushed out by the soaring real estate prices.

IBuyers believe that selling to investors allows for them to service more customers. It also helps them achieve economies of scale which helps them pass down better prices to home-sellers. For single-family landlords, working with tech companies lets them boost supply and satisfy the rampant demand that’s driving up rents.

“There’s demand for rental housing and there’s not enough of it,” said David Howard, executive director of the National Rental Home Council, a trade group. “Whatever companies can do to bring more supply online, I think that’s a good thing for rental housing and it’s absolutely good for renters.”

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