TRAVERSE CITY, Michigan: Voters across the United States received anonymous robocalls in the days and weeks before Election Day urging them to "stay safe and stay home” - an ominous warning that election experts said could be an effort to scare voters into sitting out the election.

The FBI is investigating calls that seek to discourage people from voting, a senior official at the Department of Homeland Security told reporters on Nov 3. Authorities wouldn't offer details.

The brief calls, which featured a computerised female voice, made no mention of the election. But given the lack of details, and the timing, the message was clear, according to Dan Doughty, a Kansas City resident who received the robocall on the morning of Nov 3.

"It felt like voter suppression,” Doughty told The Associated Press. "It was even so vague that whoever did it could say it was about Covid.”

The automated calls were placed to residents in nearly 90% of US area codes, according to an analysis by YouMail, a tech company that makes anti-robocall software. YouMail's findings were reported on Nov 3 in The Washington Post.

YouMail found that the calls began over the summer but increased dramatically in recent weeks. In October alone there were 10 million calls, according to Alex Quilici, YouMail’s chief executive. On Nov 3, they were reported in states including Iowa, Kansas, Florida, Nebraska and Missouri.

"This is just a test call," the voice says in one version of the robocall. "Time to stay home. Stay safe and stay home.”

Whoever created the robocalls used sophisticated tactics, routing the calls in a way that masked their identity and location, Quilici said. And unlike most robocalls, the message didn't ask for personal information or urge respondents to take any particular action - suggesting confusion and fear may have been the goal.

Those could all be clues as to who is behind the effort, he said.

"It's not a 12-year-old with a computer. This feels like maybe it's state sponsored," Quilici said. "If you want to cause mischief, you want to do it to everybody.”

In a statement, the FBI said "We are aware of reports of robocalls and have no further comment. As a reminder, the FBI encourages the American public to verify any election and voting information they may receive through their local election officials.”

The Homeland Security official, who spoke on the condition that they not be further identified, cautioned that "robocalls of this nature happen every election” and advised people to "keep calm, vote on.”

Officials warned of a separate spate of robocalls made on Nov 3 to voters in Flint, Michigan, suggesting falsely that they could wait until Nov 4 to vote in person because of long lines at polling places on Nov 3. In fact, Nov 3 is the last day to vote.

"Obviously this is FALSE and an effort to suppress the vote," Michigan Attorney General Dana Nessel tweeted. "No long lines and today is the last day to vote. Don’t believe the lies! Have your voice heard!”

Michigan is a critical swing state that President Donald Trump and former Vice President Joe Biden have both targeted in the campaign’s final days. Flint is a heavily Democratic city and the majority of its residents are Black.

Nessel filed charges on Oct 1 against two political operatives accused of orchestrating a series of robocalls "aimed at suppressing the vote” in the general election. The calls went to nearly 12,000 Detroit-area residents, she said.

Those calls falsely claimed that voting by mail would allow personal information to become part of a database used by police and creditors to trace people for mandatory vaccines, Nessel said.

Associated Press writer Frank Bajak contributed to this report.

 - AP