Veritas Investments threatened potential legal action against a tenant organizing group over what the company called a “flagrant” confidentiality breach tied to its efforts to sell off 76 apartment buildings in the city.

A lawyer for Veritas, one of the largest landlords in San Francisco, sent a strongly worded letter Monday to the Housing Rights Committee of San Francisco’s executive director, Sarah Sherburn-Zimmer, demanding that the group surrender any confidential information about the sale of the buildings and reveal who gave it to them — by 5 p.m. Tuesday.

This month, the committee posted a list of addresses for the Veritas buildings up for sale on its Facebook page, but how the group got that information remains unclear.

Veritas put the 76 buildings up for sale in late December, and under city law, the company had to first offer the buildings to a group of nonprofit housing organizations. San Francisco’s Community Opportunity to Purchase Act, which went into effect last year, is meant to create and preserve affordable housing by giving nonprofits the first chance to buy up new buildings.

In doing so, Veritas said it shared sensitive information about the tenants living in the buildings up for sale, including address, phone numbers, email addresses, the names of roommates and spouses and the amount of rent they pay. City law requires that information be kept strictly between the sellers and the interested buyers.

Nonprofits have 30 days to decide whether to submit a bid for the properties, but that time elapsed, and Veritas began marketing the buildings to other buyers. Supervisor Dean Preston has asked the company to wait another 60 days to allow city officials and the nonprofits more time to consider if some buildings can be transformed into affordable housing. Veritas has said it’s amenable to working with the city.

Sherburn-Zimmer, who could not be reached for comment Monday, told The Chronicle last week that the committee was trying to alert tenants in affected Veritas buildings and inform them of their rights as renters.

“There is no violation that I know of in how I got the information,” she said Friday. “I think it’s crucial that tenants have the right to protect themselves.”

Veritas’ attorney, Jonathan Sommer, wrote that “it is no defense that HRC is the recipient of misappropriated information versus the leaker of that information. You knew you were aiding and abetting the unlawful disclosure of information that is confidential by law.”

The company, Sommer wrote, “will take further action as it deems appropriate to protect its rights, the rights of its tenants and the confidential information.”

In addition to disclosing and returning all of the confidential information it might possess, Veritas also asked the committee to scrub any social media posts that share the information.

The Housing Rights Committee and Veritas have clashed repeatedly over the years. The committee has accused Veritas of harassing longtime tenants in an effort to boot them from their apartments in order to jack up rents. Veritas has denied those claims.

A lawsuit brought by 68 Veritas tenants making similar claims was filed in 2018. A San Francisco Superior Court Judge tossed out the case for a second time last November, but gave the tenants until the end of this month to amend their complaint.

— Dominic Fracassa

Dominic Fracassa is a San Francisco Chronicle staff writer. Email: dfracassa@sfchronicle.com Twitter: @dominicfracassa