Democratic lawmakers have claimed there were some working-class and primarily Latino neighborhoods that had no polling places.
Chris Herren, chief of the Justice Department's Voting Section, sent a letter to Maricopa County Recorder Helen Purcell after voters complained of waiting for hours to vote due to an insufficient number of polling stations.
"We have received reports that a number of Maricopa County voters waited several hours to cast a ballot on election day," said Herren's letter,
which was sent late last week. "We also understand that there were allegations of disproportionate burden in waiting times to vote on election day in some areas with substantial racial or language minority populations."
Justice Department officials have requested that Purcell send them information by April 22 on various information, including a complete list of registered voters, data on voters and staff at each polling place location and county procedures for recording political party registration.
A spokeswoman for Purcell, a Republican, told CNN Tuesday they were cooperating with the federal government.
"We are fully cooperating with the Department of Justice. And we are in the process of gathering all of the information that they have asked for," Elizabeth Bartholomew said.
But Bartholomew said any suggestion that Purcell intentionally suppressed voters is false.
"Helen's career has been focused on trying to make voting easier and more accessible. There was absolutely no way she tried to make it harder for voters to vote or to suppress anyone from voting," she said. "At the end of the day, it was just a huge miscalculation and a mistake. And we're moving forward and making changes to make sure that that never happens again."
Phoenix Mayor Greg Stanton, a Democrat, as well as Arizona Democratic Rep. Ruben Gallego and other lawmakers, called for investigations into race- and class-based voter suppression into Arizona's largest county soon after the primary, which was held on March 22.
Maricopa County had 60 polling stations. There were at least 200 polling stations in that jurisdiction in 2012, but Republican officials said they decreased the number to save money.
Even after waiting in line for hours, some people were not allowed to vote. At least 20 Democratic voters contacted the Arizona Democratic Party to say that when they arrived at the polls, they were told that they were registered as independents and therefore unable to vote in the closed primary.
Arizona Gov. Doug Ducey, a Republican, said the situation was "unacceptable."
"If people want to take the time to vote they should be able to, and their vote should be counted," he tweeted the day after the primary, calling on election officials to figure out what went wrong.
Purcell initially defended the closings, telling Fox 10 Phoenix
that voters could have voted via early ballots rather than risking waiting in line. But she later took "full responsibility for what happened" at polling stations while speaking at a Maricopa County Board of Supervisors meeting.
Historically, the Department of Justice had to approve any changes in voting procedure in Arizona because of the state's history of discrimination against minorities in voting. But a Supreme Court decision in 2013 allowed Arizona to begin making changes without federal oversight. Leaders said if federal scrutiny had been required before making changes, Tuesday's situation would have never happened.
Vermont Sen. Bernie Sanders, who lost Arizona to Democratic front-runner Hillary Clinton, called the situation "a disgrace" soon after the primary.
"In the United States of America, democracy is the foundation of our way of life," the Democratic presidential hopeful previously said. "And what happened in Arizona is a disgrace. I hope that every state in this country learns from that and learns how to put together a proper election where people can come in and vote in a timely manner and go back to work."