Attorney General Maura Healey is calling for an investigation into the opaque regulatory process that she charges is costing Massachusetts customers millions and generating higher utility company profits than those in neighboring states, as a major Bay State utility prepares its case to boost rates next month.

In a letter sent this week to the Department of Public Utilities, Healey asked for an investigation into why Massachusetts electric companies are charging millions for rate hikes that go toward higher shareholder profits than companies in other states, and she called for “a comprehensive and public review of utilities’ allowed profits … to bring more clarity and openness to the rate-setting process.”

In October, the DPU allowed a 9.9 percent profit with National Grid’s recent $101 million rate hike — the highest rate of return for an electric or gas utility company in Massachusetts since 2009, the AG’s office said. Connecticut regulators last week limited profits for United Illuminating to 9.1 percent.

If Bay State regulators had set a similar 9.1 percent rate for National Grid, it would save ratepayers $12 million, the AG’s office said

“Massachusetts customers should not be paying millions more towards utility profits than customers in neighboring states,” Healey said. “As the ratepayer advocate for the state, we must ensure best practices and a transparent process that is understandable to the public.”

Rates for electric distribution companies like National Grid and Eversource Energy are set by the DPU, which determines how much profit the company can collect on the millions it invests in poles, wires and electrical systems it uses to deliver power.

Eversource Electric has announced it will be asking the DPU to approve rate increases in January 2017.