The top Senate Democrat on the congressional Joint Economic Committee launched a new probe Monday into investment firms that hold large stakes in mobile home parks.

Sen. Maggie Hassan of New Hampshire has asked six firms to produce internal documents and materials showing the impact their business practices have had on mobile home residents and the profits their investments have generated.

About 22 million people in the U.S. live in mobile home communities across the country, and in recent years, investment companies and private equity firms have bought up many of the parks.

The firms that received letters include some of the biggest players in the industry: Alden Global Capital, owner of Homes of America; the BoaVida Group; Legacy Communities; Patriot Holdings; Philips International; and Sun Communities.

Hassan has also requested information about lease agreements, rents and fees charged, evictions, maintenance and repair histories, numbers of complaints filed by residents and settlements of lawsuits with residents.

None of the six firms responded to a request for comment on the letters.

Residents of mobile home communities are often vulnerable, Hassan’s letters notes; many are seniors, families on limited incomes and people with disabilities. They can find it very difficult to move if their rents rise significantly, or if their lease terms change.

Unlike traditional renters, many mobile home residents own their homes but lease the land they sit on. If the rental costs increase dramatically, they could be forced to move their entire homes to new locations, which typically involves significant costs and risks structural damage.

Hassan has also asked for details of ownership stakes the firms hold in the communities and how they have been financed.

“The thousands of Granite Staters and millions of Americans who live in mobile home parks deserve fair rent, safe living conditions, and the ability to protect themselves from mistreatment,” Hassan told NBC News in a statement.

“As corporate investment firms buy up more and more of these communities, they still have a responsibility to meet these basic standards for each and every one of their residents,” she said.

Hassan notes the rise in the purchase of manufactured housing by investment firms, which reached almost $10 billion in 2021. According to an analysis cited in the letters, investment firms accounted for almost one-quarter of the buyers of mobile home parks over the previous 24 months.

Mobile home communities can offer cheaper housing compared with alternatives like apartments and single-family homes. The degree to which investment firm buyers raise rents and fees associated with living in mobile home communities may exacerbate what is already an affordability crisis for U.S. homeowners.

Jim Baker is executive director of the Private Equity Stakeholder Project, a nonprofit watchdog organization that analyzes the impact the industry has on health care, housing, climate and jobs. His group has reported on private equity’s investments in manufactured housing, and he said he welcomed Hassan’s investigation.

“Mobile home residents across the country have reported rent hikes as high as 100%, exorbitant junk fees, poorly maintained water systems and facilities, and unjust and aggressive evictions,” he said in a statement. “Put simply, wealthy investors are squeezing every last cent from some of our most vulnerable and least-resourced communities in the name of profit.”

Hassan asked the companies to provide documents and answers to her questions by Jan. 5.