Staring down an election without precedent, Minnesota officials entered the final push to ensure voters' ballots could be counted amid a historic surge in mail voting, legal challenges, federal monitors and elevated fears of conflict at polling places.

Secretary of State Steve Simon said Monday that a record 1,839,710 absentee ballots have been returned and accepted — roughly 62% of the total turnout for 2016. Another 297,482 requested absentee ballots remained outstanding. The office provided those figures late Monday after in-person voters completed their ballots after waiting up to three hours at early voting sites that closed at 5 p.m.

The polls opened at 7 a.m. for Election Day voting in Minnesota.

As voters rushed to return absentee ballots, the U.S. Justice Department announced Monday that federal officials are being sent to monitor for potential election law violations in Minneapolis as part of an 18-state Election Day operation.

Minnesota U.S. Attorney Erica MacDonald had previously said her office would appoint a prosecutor to take complaints of possible federal election law violations. A Justice Department spokesperson said Monday that the additional 13 monitors consist of a group of paralegals in the U.S. Attorney's Office's civil division in Minnesota. Another monitor is from the Justice Department's disability rights section. They will visit multiple sites in Minneapolis but will only be allowed to watch from the outside of polling places, the spokesperson said.

The federal monitors will be watching for and documenting any cases of voter intimidation or lack of access for voters with disabilities. The department did a similar sweep for the general election in 2016, fanning out across 28 states, but did not go to Minnesota. The last time federal monitors observed an election in Minneapolis was 2004, according to Simon's office.

City of Minneapolis spokeswoman Sarah McKenzie said the city was aware of the Justice Department plan and appreciated that the monitors "will be located outside of the 100-foot buffer zone, as allowed under Minnesota election law" and would contact the city with any concerns observed in the field.

Monday marked the final day of early voting in the general election that began Sept. 18 — tied with South Dakota for the longest early voting period. The crush of mail-in ballots reflected concerns about the COVID-19 pandemic that upended planning for the 2020 election earlier this year. Potential mail delays also sent voters rushing for absentee ballots for early voting.

Adding to voters' concerns, the Eighth U.S. Circuit Court of Appeals ordered on Thursday that all mail-in ballots received after 8 p.m. on Election Day be segregated for possible legal challenges. An earlier state court agreement had expanded the deadline for receiving mail ballots to Nov. 10, if they were postmarked by Election Day.

With time running out, Simon's office is not appealing the federal ruling, which he described Monday as "unnecessarily disruptive." Simon noted the possibility of additional litigation surrounding the fate of any ballots that arrive after 8 p.m. Tuesday.

Since late Thursday, state officials have been urging Minnesotans to stop mailing in ballots and instead vote in person, either at early voting centers or at the polls on Tuesday. Those dropping off absentee ballots must do so in person by 3 p.m. on Election Day.

Also Monday, Justice Neil Gorsuch denied a request for the U.S. Supreme Court to take up a challenge to Minnesota's mandate that voters wear masks at polling sites. The petition was filed by a conservative activist group that unsuccessfully sought in lower courts to bar the mandate.

By the end of Monday, early voters at the Hennepin County Government Center in Minneapolis faced a three-hour wait to cast their ballots. Elsewhere in the city and outside the metro, voters stood for an hour or more at their election centers. When first-time early voter Liban Hussein got in line in Minneapolis, he said he was asked by election workers if he would be able to stand there for up to two hours.

"It's crazy. It doesn't even look like the line is moving," said Hussein, 45. "Thank God the sun is shining."

More than 60% of Minneapolis voters cast their ballots during the early voting period, with more than 166,900 ballots processed as of Monday, said Minneapolis Elections & Voter Services director Grace Wachlarowicz.

The race to make sure Minnesota's election goes smoothly reflects a year in which election workers were met by a new, unseen adversary in a global pandemic. Entering the year, state and federal officials were on high alert for a redux of 2016, when Russian state actors waged a broad systemic campaign to interfere in the presidential election and try to penetrate election systems nationwide, including in Minnesota.

The ideological intensity of the 2020 election is also reviving fears of resurgent civil unrest on par with what followed George Floyd's death in May. Minnesota Attorney General Keith Ellison intervened amid reports last month that private security contractors were advertising jobs for armed ex-special forces members to illegally guard polling places in Minneapolis. The Tennessee-based Atlas Aegis, facing a federal civil suit and a probe by Ellison's office, has since assured state officials that it erroneously advertised security jobs intended for private businesses as being geared toward polling sites.

The Atlas Aegis postings prompted Ellison to sound the alarm over concerns about voter intimidation. Simon's office meanwhile issued a reminder that state law allows only for designated poll "challengers" to keep a distance between voters and only report concerns to election judges in writing.

Reflecting the focus on election administration in Minnesota, Gov. Tim Walz's public daily schedule for Election Day eve listed only a phone call with Simon.

Simon meanwhile told reporters earlier Monday that he would not be surprised to see the state "smash" the modern-day record for voter turnout of 77% set in 2008.

"All I know is it's pretty electric out there right now," Simon said. "On all sides, by the way. Equal opportunity electricity, I think, on the left, right, red, blue. People are fired up to vote."

Staff writers Zoë Jackson, Liz Navratil and Tim Harlow contributed to this report.