METHUEN, Mass. -- A Methuen dialysis clinic apologized Monday, two days after an 86-year-old woman was accidentally forgotten and locked inside the facility.

Maureen Perry, who is temporarily bedridden, was at the Fresenius Dialysis Clinic on Merrimack Street in Methuen Saturday night.

Several hours after staff left for the night, Pam Crosby learned that her grandmother was still inside after being apparently forgotten.

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After discovering the mix-up Saturday night, Crosby called 911. Firefighters responded and transported Perry to her rehab facility.

On Monday, CBS Boston reports that Fresenius Medical Care North America said a staff member who closed the clinic "mistakenly believed" that all patients had left.

In a statement, the company said: "Unfortunately, one of our patients remained in her dialysis chair at the time. She was later safely returned to her residence. We deeply regret what occurred and have spoken with the patient's family. Our patients' care and safety are our top priorities and we have initiated additional processes and safeguards to ensure that an incident such as this does not happen again. We apologize to the patient, her family, and the entire community."

Ronald Quaranto of Cataldo Ambulance Service said one of the company's ambulances was called Saturday night to pick up Perry following her dialysis. When the ambulance driver arrived, the lights were dark at the clinic.

"They immediately notified our dispatch center that the facility appears to be locked and there's no one there," said Quaranto. "Our dispatch center then called the dialysis center to see if there was anyone there inside. There was no answer."

Following the incident, Methuen police contacted the Department of Elder Affairs.

Because it was not a case of neglect or abuse, the agency declined to investigate.

The Department of Public Health, however, is looking into the incident.

"DPH is conducting a thorough investigation into this situation," said Interim Director of Communications Scott Zoback.