The lawyers investigating Facebook operating chief Sheryl Sandberg’s use of corporate resources are examining behavior going back several years, said people familiar with the matter, focusing on the extent to which staffers worked on her personal projects.

A number of employees have been interviewed as part of the investigation by Facebook parent Meta Platforms META, -4.58% the people said, adding that the review has been under way since at least last fall.

It includes an examination of the work Facebook employees did to support her foundation, Lean In, which advocates for women in the workplace, as well as the writing and promotion of her second book “Option B: Facing Adversity, Building Resilience, and Finding Joy,” which focused on her grieving process following the sudden death of her husband, Dave Goldberg, in 2015, the people said.

The Wall Street Journal previously reported that the investigation included a review of Ms. Sandberg’s use of corporate resources to help plan her coming wedding. That is a small piece of the investigation, according to the people familiar with the matter, who said it involves a broader review of Ms. Sandberg’s personal use of Facebook’s resources over many years.

Ms. Sandberg, 52 years old, announced last week she was resigning from her day-to-day role after 14 years, though she said she would continue to serve on the board of directors. Ms. Sandberg has been the longtime lieutenant to Chief Executive Mark Zuckerberg, and in that role—and as the author of the leadership book “Lean In”—became one of the most prominent women in business. Ms. Sandberg said she was looking forward to spending more time on her foundation and women’s issues.

Ms. Sandberg has told friends and co-workers that she decided to step down because she was burned-out and weary of continuing her role as a “punching bag” for Meta’s critics. She also sees Mr. Zuckerberg’s pivot to the so-called metaverse as a multiyear project that she wasn’t eager to take on, not least because it doesn’t directly entail the use of her core strengths in building advertising businesses.

People close to Ms. Sandberg say that while the review has irked her in recent months, it played no role in her decision to leave the company later this year. “Sheryl did not inappropriately use company resources in connection with the planning of her wedding,” a spokeswoman for Ms. Sandberg said last week. A Meta spokeswoman declined to comment for this article.

In 2021, Facebook paid nearly $9 million for Ms. Sandberg’s security at her homes and during personal travel and $2.3 million for costs related to her personal use of private planes, according to the company’s most recent proxy. Facebook spent $15.2 million on Mr. Zuckerberg’s security and $1.6 million in private plane costs.

Facebook employees also worked on Mr. Zuckerberg’s projects, including his 2017 tour of 30 American states, according to public records. One stop in Glacier National Park in Montana, Mr. Zuckerberg was joined by at least three full-time Facebook employees, according to public records.

An expanded version of this story appears on WSJ.com.