The letter went out to about 1,900 Californians a few weeks ago from law firms bringing a class-action suit against one of the country’s largest assisted-living chains.

If the recipients, or their family members, had lived in a community operated by Sunrise Senior Living in recent years, “we would like to speak with you regarding your residency and experience,” the letter said.

It was the latest action in an ongoing campaign: Since 2013, a group of law firms has systematically sued several major chains operating in the state, employing an unusual strategy.

“It all boils down to the use of assessments, or lack thereof,” said Kathryn Stebner, the trial counsel in the case. “These are à la carte facilities — the more needs you have, the more you have to pay. So, they assess you.”