Prompted by repeated consumer complaints, state Attorney General William Tong launched an investigation Monday into slow internet speeds, mediocre technical responses and hidden fees by the Altice Optimum cable company.

Under the Connecticut Unfair Trade Practices Act, Tong will investigate nearly 500 complaints by consumers over a period of years.

One of the most common complaints is consumers are protesting that they are not getting the fast internet speeds that they paid for. Some consumers who signed up for 300 or 400 megabits per second found out that they consistently got only 120 megabits per second, Tong said. One customer on the 400 megabits plan said she received only 10 to 40 megabits per second, Tong said.

“We received complaints from numerous consumers who ran speed tests. These are people that are much more technically savvy than I am,’' Tong said at a press conference in Hartford. “They discovered that they are getting much lower speeds than they paid for. And when they call Optimum looking for help — if they can get through to somebody — their problems are not resolved.’'

A longtime Stamford resident, Tong said he personally is an Altice Optimum customer, which has a wide footprint in Fairfield County. Tong said the company had “not yet’' responded regarding the investigation.

“This should come as no surprise that customers are unhappy,’’ Tong told reporters. “We’ve learned, as consumers, to put up with some amount of poor service from some of our cable providers because you call up and you’re on hold and the call gets dropped. Then the call gets transferred, and it gets dropped. Many of us have had that experience personally, but this goes well beyond some of those constant complaints and refrains. This is people who really need these services to work and go to school and manage their lives. The internet is so important in every facet of our lives.’’

After multiple calls, Optimum officials could not be reached for comment Monday at their Long Island headquarters.

Technician visits to the home can cost $75, but the complaints are continuing, Tong said.

“Customers have the right to receive and expect to receive the internet speeds that they paid for,’’ Tong said.

Besides problems with speeds, Tong’s office has handled complaints about customer service, technical support, and service fees.

“Our investigation seeks comprehensive records dating back to January 2017 to determine exactly what Altice Optimum knew and what they were doing to deliver the internet speeds and service they promised,’’ Tong said. “If our investigation finds that Optimum violated Connecticut law, we will not hesitate to hold them accountable.’’

The broader problem, officials said, is that consistent internet access has become a necessity across the state and is often as important as services provided by public utilities.

In a similar case, Tong’s office reached a settlement with Frontier Communications in August that had been prompted by more than 1,400 complaints — nearly three times the number for Optimum. Some of the issues were the same, such as mediocre customer service, poor internet quality, high charges, and problems with equipment returns.

“Frontier Communications, which also had a reputation, in fact did not deliver on the promised service,’' Tong said.

Frontier settled the civil case for more than $60 million, and the agreement is designed to “dramatically expand access to high-speed internet for Frontier customers in economically distressed communities, end a hidden monthly $6.99 internet surcharge, and force significant improvements in Frontier’s marketing and customer service,’' Tong said.

State Sen. Norm Needleman, an Essex Democrat who co-chairs the legislature’s energy and technology committee, said at the news conference that he is concerned about companies that are “over-promising and under-delivering without any explanation. ... All of them have an obligation to their customers, whether you classify them as ratepayers or not.’'

Needleman added, “As the internet has become a vital part of everyday life, households relying on these services for employment, education and entertainment can experience significant harm if they lack reliable internet service. If Altice Optimum contributed to that harm, they should face the consequences.’'

Christopher Keating can be reached at ckeating@courant.com