(Barta IV via Flickr)

New York officials are investigating some of the nation's biggest Internet providers over claims that customers aren't getting the Internet speeds they've paid for.

In letters to Verizon, Time Warner Cable and Cablevision on Monday, the New York attorney general's office cited "technical and business decisions" by the companies that may be leading to reduced download speeds for individual customers. The letters also demand that the companies provide internal company data to the government to prove that consumers are getting the advertised speeds.

"What they're doing is sending a shot across the bow at industry and saying, 'If you're making these claims, you'd better be able to substantiate them,' " said David Vladeck, a former director of the Federal Trade Commission's consumer protection bureau.

The three Internet providers vowed to cooperate with the probe, and all said in statements Monday that they were confident in the speeds they provide to end-users.

Officials were alerted to a potential problem amid a slew of consumer complaints and public data from third-party researchers. Internet providers have performed ably on some speed tests, such as the Federal Communications Commission's Measuring Broadband America report. The most recent of those white papers showed that carriers largely met their advertised speeds.

But the FCC's research, along with many off-the-shelf online speed testers, doesn't measure everything that goes into determining consumer speeds, said Tim Wu, a senior enforcement lawyer in the attorney general's office who wrote to the providers Monday. (Wu is also known for coining the phrase "net neutrality" and running for New York lieutenant governor in 2014.)

The tests typically measure the speed at which data flows from one point to another across an Internet provider's own network. But they mostly ignore the rate at which data from the rest of the Internet actually enters the carrier's network, said Wu. If there isn't enough capacity at that point, known as the point of interconnection, then consumers could experience delays as Netflix videos and other content struggle to make it into a provider's systems.

When a company such as Time Warner Cable advertises speeds of, say, 300 megabits per second, "they don't promise you'll have 300 Mbps to Time Warner Cable — they promise 300 Mbps to the Internet," said Wu in an interview. "That requires going through interconnection. And that's a bottleneck they control and can substantially affect the speeds the consumer experiences."

Netflix subscribers may remember their outrage when a flood of video traffic last year got clogged up at the edge of networks belonging to Comcast, Verizon and other Internet providers. The result was a series of commercial deals that involved Netflix paying the companies for extra capacity at the point of interconnection.

Now, according to the New York attorney general's office, those types of deals — and perhaps even those that don't involve money changing hands — may be insufficient to guarantee consumers' advertised speeds.

The companies will have until Nov. 8 to respond to the state government's inquiry.