(CNN) Michigan officials may have altered sample data to lower the city of Flint's water lead-level reports, according to official documents and a researcher who conducted extensive tests there over the last six months.

Documents and emails show discrepancies between two reports detailing the toxicity of lead samples collected by the Michigan Department of Environmental Quality and the city of Flint between January and June 2015, Professor Marc Edwards from Virginia Tech University said.

Edwards is the lead researcher for the Flint Water Study, a research group that has conducted numerous tests on Flint's system and was the first to publicly identify high levels of lead in the water.

The documents and emails were released by researchers from Virginia Polytechnic Institute and State University through a Freedom of Information Act request and viewed by CNN.

According to Edwards, the Michigan Department of Environmental Quality and the city of Flint collected 71 lead level samples from homes when they were required to collect 100. The final report from the Department of Environmental Quality however, only accounted for 69 of those 71 samples.

Edwards said those two discarded samples were "high-lead" and would have lifted the "action level" above 15 parts per billion. The public must be alerted and additional action must be taken if lead concentrations exceed an "action level" of 15 parts per billion in drinking water, according to the Environmental Protection Agency's website.

The report says the two discarded samples were removed from the list "for not meeting sample criteria." In a statement, the Department of Environmental Quality says the rationale for dropping the samples from these two sites "appropriately met the requirements for invalidating samples per federal regulation."

Edwards said the samples should have been taken from homes with lead pipes. The reports say they were, but Michael Glasgow, then-assistant supervisor of the Flint water plant, said this is not true. Glasgow told CNN the records were not complete, and the sampling teams did not know which homes had lead pipes.

"In essence, the state took an 'F-grade' for Flint water's report on lead and made it into an 'A-grade,'" Edwards told CNN.

Federal law requires testing of lead and copper levels in drinking water based on a procedure called the Lead and Copper Rule, or LCR.

The lead-level report was released in summer 2015, at a time when state and federal officials became increasingly concerned by reports of contaminated water in Flint, emails show.

A June 2015 leaked internal memo from the Environmental Protection Agency raised the issue, saying high lead results in the drinking water were a "serious concern."

The memo, written by an EPA water expert, went on to note that several of the city of Flint's sampling procedures were flawed and data might therefore be compromised.

Of particular concern was the alleged use of a coagulant in the water that may have increased lead levels, and the alleged lack of corrosion control treatment. The coagulant, ferric chloride, was originally used to "improve the removal of organic matter" in the water, but studies showed it may have adverse effects, according to the memo.

The memo notes that residents were instructed to "pre-flush" taps before samples were taken, a practice that has been shown "to result in the minimization of lead capture and significant underestimation of lead levels."

After then-Flint Mayor Dayne Walling was alerted to the memo, EPA officials told him the findings were a "preliminary draft." The final draft was released four months later, in November. Walling lost his recent re-election bid in a campaign centered around the issue.

In a statement, the EPA says the memo contained "confidential personal and enforcement-sensitive information," and that the memo was immediately circulated to its regional staff working on the Flint water issue.

Receiving community concerns and criticism the time, Republican Gov. Rick Snyder's office reached out to state officials about the emerging reports.

A former spokesman for Snyder wrote to Michigan Department of Health and Human Services officials in July 2015, "I'm frustrated by the water issue in Flint."

"These folks are scared and worried about the health impacts and they are basically getting blown off by us."

Follow-up emails show officials working to address the issue.

A July 2015 email from former Michigan Department of Environmental Quality spokesman Brad Wurfel said, "...the bottom line is that residents of Flint do not need to worry about lead in their water supply."

Wurfel went on to say, "...DEQ's recent sampling does not indicate an eminent health threat from lead or copper."

"This is definitely being driven by a little science and a lot of politics," Wes Priem, an Industrial Hygiene Manager with the Healthy Homes Section of the Michigan Department of Health and Human Services, said in September 2015.

In preparation for a late-September 2015 press conference and in response to data released by Edwards, Michigan Department of Health and Human Services Director Nick Lyon emailed fellow staff, "I would like to make a strong statement with a demonstration of proof that the lead blood levels seen are not out of the ordinary and are attributable to seasonal fluctuations."

Jennifer Eisner, public information officer for the Michigan DHHS, said the emails reflect the fact that at that time, the state's analysis attributed the increase to seasonality.

"It wasn't until later that MDHHS epidemiologists took a more in-depth look at the data by ZIP code and confirmed an increase outside of normal trends," Eisner said in a statement.

Residents and elected officials have often placed the blame squarely on Snyder's shoulders.

"It's pretty hard to avoid the fact that all of this has been on the watch of the governor," said Michigan U.S. Rep. Dan Kildee, a Democrat.

"I'm really trying to focus on holding the people who made the decision accountable, not just apologize."

The governor's office said it responded "aggressively and quickly" to address the challenges in Flint.

"We are now working very closely with local leaders and are reaching out to researchers who have been involved with the issue [so] we can work together on making sure Flint residents have access to safe, clean water," Dave Murray, press secretary for Snyder, told CNN.

On Tuesday, Snyder signed an executive order activating the Michigan National Guard to inform residents about the availability of filters, bottled water, water testing kits and information on lead.

The guard will be going door-to-door to ensure residents have access to safe water and have their water tested, according to the executive order.

This comes after the State of Emergency that Snyder declared on January 5.