LONDON - After the outbreak of the water contamination crisis in the US city of Flint, Michigan, more US cities have been exposed to have similar problems, according to an investigation by The Guardian newspaper.

At least 33 cities across 17 US states have used water testing "cheats" that potentially conceal dangerous levels of lead over the past decade, the British newspaper reported on Thursday.

Testing methods that can avoid detecting lead include asking testers to run faucets before the test period, known as "pre-flushing;" to remove faucet filters, called "aerators;" and to slowly fill sample bottles.

The investigation found that big cities like Chicago, Boston, Philadelphia, Detroit and Milwaukee were among the cities using poor testing methods.

For 25 years, the Environmental Protection Agency (EPA) has required water utilities to test a small pool of households for lead contamination at least every three years.

But the way residents are instructed to sample their water, as well as which households are chosen for testing, can profoundly impact how much lead is detected, said the report.

Two states, Michigan and New Hampshire, advised water departments to give themselves extra time to complete tests so that if lead contamination exceeded federal limits, officials could re-sample and remove results with high lead levels, it said.

Marc Edwards, a Virginia Tech scientist who first uncovered the crisis in Flint, described water testing in some of America's largest cities as an "outrage."

"They make lead in water low when collecting samples from EPA compliance, even as it poisons kids who drink the water," Edwards said. "Clearly, the cheating and lax enforcement are needlessly harming children all over the United States."

Flint, a city northwest of Detroit in Michigan, switched off Detroit's Lake Huron water supply in April 2014 and turned to the Flint River as an interim source while a new regional pipeline was being built.

Residents immediately complained about the water's color and odor, and independent investigators discovered elevated lead levels in the water and blood of children.

The State of Michigan initially downplayed people's concerns, but eventually confirmed finding lead in Flint water supply in October 2015 and began taking steps to address the crisis.

As a result, the state Department of Environmental Quality director and a department spokesman resigned and another official has been fired. Michigan Governor Rick Snyder called for an investigation into the state Health Department.

US Attorney's Office in Detroit started investigating into Flint water crisis in January this year, along with the EPA.

Eastern US states are considered to have a high risk of lead contamination due to their aging infrastructure.