Against the green hills of Thousand Oaks’ preserved open space, executives from Atara Biotherapeutics Inc. gathered Wednesday morning with developers, bankers, politicians and commercial real estate brokers to celebrate the groundbreaking of the drug maker’s new facility.

The city announced in early February that Atara signed a 15-year lease for nearly 91,000 square feet, the second-largest of nine industrial buildings to be built in the Conejo Spectrum Business Park on Conejo Spectrum Street, which is between Rancho Conejo Boulevard and Conejo Center Drive.

Irvine investment and development firm Sares Regis Group and a financial partner bought the last 29 acres of buildable land in the park late last year. Colliers International brokered the deal out of its Encino office, and Senior Executive Vice Presidents John DeGrinis and Patrick DuRoss, as well as the firm’s Jeff Abraham, negotiated the sale, and are handling the leasing.

The nine buildings will total 500,000 square feet, and range from 37,000 square feet to 100,000 square feet. Atara will operate at 2430 Conejo Spectrum St.

Atara, a clinical-stage biopharmaceutical company headquartered in South San Francisco, currently operates its research and development division on Park Terrace Drive in Westlake Village. It plans to move its research and development division to the park and expand manufacturing to bring it all in-house.

Peter Rooney, president of Sares Regis, talked at the event about how the company, which bought the property with financial partner JPMorgan Chase & Co., plans to hold the buildings for 20 to 30 years rather than sell them, as developers often do soon after buying land and building properties on them.

He said the companies checked into what Atara did and its financial backing when hearing about its interest in leasing the space. Rooney didn’t go into the details of who its investors are.

“We don’t understand what you do, but we know who’s behind you,” Rooney said.

Gad Soffer, executive vice president with Atara, said the company’s plans for the new site include manufacturing, research and development, regulatory affairs and management of clinical trials.

“It’s exciting to be able to bring manufacturing capabilities in-house and be able to make the transition to the next stage of the company,” he said.

The property sits less than a mile from the 101 Freeway which provides access to the 5, 23 and 118 freeways.