A group that says it represents about 3,500 Central Maine Power customers has issued a letter to state officials demanding a more thorough investigation into customer complaints of inflated CMP electric bills.

Calling itself CMP Ratepayers Unite, the group said it sent the letter because of concerns that the Maine Public Utilities Commission’s ongoing fact-finding probe into the matter has raised more questions than it has answered. The group also said Maine officials need to provide “immediate relief” to CMP customers who believe they have been overcharged by the utility while the PUC probe continues.

Many CMP customers are furious over a revelation that the company was not forthright about serious technical problems with its new billing system at a time when more than 1,500 customers were complaining of inexplicably high electric bills.

CMP has nearly 550,000 customers in Maine. About 97,000 CMP customers saw their monthly bills increase by 50 percent or more in December, January or February over the same month a year earlier, according to information the power company has provided to state regulators.

Many customers have said their bills suddenly doubled or tripled after CMP switched to the new billing system in late October. They also have complained of other problems, such as receiving multiple bills or no bills, and being erroneously dropped from the utility’s monthly auto-payment system.

CMP has acknowledged problems with the new system but has said it did not cause customers to be overcharged. The company said the likely causes were nearly a month of sub-freezing temperatures in December and January, combined with an 18 percent hike in the standard offer rate for electricity.

A copy of the group’s letter sent to the Portland Press Herald is addressed to several state officials including PUC Administrative Director Harry Lanphear, Maine Attorney General Janet Mills and Gov. Paul LePage, whom it incorrectly calls “John LePage.”

The letter includes more than 40 specific complaints relating to CMP’s billing, metering, customer service and other issues.

This story will be updated.