An AP Investigation reveals water pollution from oil drilling techniques like fracking are more widespread than previously believed. 2012 Katie Orlinsky

In at least four states that have nurtured the nation's energy boom, hundreds of complaints have been made about well-water contamination from oil or gas drilling, and pollution was confirmed in a number of them, according to a review that casts doubt on industry suggestions that such problems rarely happen.

The Associated Press requested data on drilling-related complaints in Pennsylvania, Ohio, West Virginia and Texas and found major differences in how the states report such problems.

Texas provided the most detail, while the other states provided only general outlines. And while the confirmed problems represent only a tiny portion of the thousands of oil and gas wells drilled each year in the U.S., the lack of detail in some state reports could help fuel public confusion and mistrust.

The AP found that Pennsylvania received 398 complaints in 2013 alleging that oil or natural gas drilling polluted or otherwise affected private water wells, compared with 499 in 2012.

The Pennsylvania complaints can include allegations of short-term diminished water flow, as well as pollution from stray gas or other substances. More than 100 cases of pollution were confirmed over the past five years.

Just hearing the total number of complaints shocked Heather McMicken, an eastern Pennsylvania homeowner who complained about water-well contamination that state officials eventually confirmed.

"Wow, I'm very surprised," said McMicken, recalling that she and her husband never knew how many other people made similar complaints, since the main source of information "was just through the grapevine."

The McMickens were one of three families that eventually reached a $1.6 million settlement with a drilling company. Heather McMicken said the state should be forthcoming with details.

Over the past 10 years, hydraulic fracturing, or fracking, has led to a boom in oil and natural gas production around the nation. It has reduced imports and led to hundreds of billions of dollars in revenue for companies and landowners, but also created pollution fears.

Extracting fuel from shale formations requires pumping hundreds of thousands of gallons of water, sand and chemicals into the ground to break apart rock and free the gas.

Some of that water, along with large quantities of existing underground water, returns to the surface, and it can contain high levels of salt, drilling chemicals, heavy metals and naturally occurring low-level radiation.

But some conventional oil and gas wells are still drilled, so the complaints about water contamination can come from them, too. Experts say the most common type of pollution involves methane, not chemicals from the drilling process.

Some people who rely on well water near drilling operations have complained about pollution, but there's been considerable confusion over how widespread such problems are.

For example, starting in 2011, the Pennsylvania Department of Environmental Protection aggressively fought efforts by the AP and other news organizations to obtain information about complaints related to drilling.

The department has argued in court filings that it does not count how many contamination "determination letters" it issues or track where they are kept in its files.