A voter in Maricopa county, Arizona, claims a group of people watching a ballot drop box photographed and followed the voter and their wife after they deposited their ballots at the box, accusing them of being “mules”.

The voter filed a complaint with the Arizona secretary of state, who forwarded it to the US Department of Justice and the Arizona attorney general’s office for investigation, according to Sophia Solis, a spokesperson with the secretary of state’s office.

The incident allegedly occurred at a Mesa, Arizona, outdoor drop box on the evening of 17 October. Early voting, both in person and via mailed ballots, began on 12 October ahead of the midterm elections.

“There’s a group of people hanging out near the ballot drop box filming and photographing my wife and I as we approached the drop box and accusing us of being a mule. They took a photographs [sic] of our license plate and of us and then followed us out the parking lot in one of their cars continuing to film,” the voter wrote in the complaint.

In Arizona, voters can only drop off ballots for themselves, people in their households or families, or people they’re providing care for. Other states don’t ban so-called ballot harvesting. The practice became illegal in Arizona in 2016.

The incident comes as people in Maricopa and Yavapai counties have started to monitor drop boxes, spurred by the movie 2000 Mules, which makes unsubstantiated claims that “mules” are stuffing ballot boxes with votes. In other states, similar efforts to monitor drop boxes are under way, organized by people who remain convinced the 2020 presidential election was stolen.

The Maricopa drop boxes are already under video surveillance by the county and broadcast on a live feed on the county’s website, and the Yavapai drop boxes have cameras mounted on them.

Election officials and voter advocacy groups have warned that the practice could lead to voter intimidation. At a press conference in Phoenix on Wednesday, Maricopa county supervisor Bill Gates said people outside the Maricopa county tabulation and election center were approaching and photographing election workers as they went into the site to work.

“They’re harassing people. They’re not helping further the interests of democracy. If these people really wanna be involved in the process, learn more about it, come be a poll worker or a poll observer,” Gates said.

On Wednesday, a few people with cameras gathered outside a fence around the tabulation center’s parking lot and identified themselves to reporters as part of a group called Clean Elections USA. On its website, the group says it’s looking for “true Patriots to take a stand and watch the drop boxes” by gathering video and witnessing any potential “ballot tampering”.

In Yavapai county, groups that planned to organize drop box watches received legal warnings that they could be intimidating voters, halting their plans for coordinated watches, but some people are still watching the boxes from their cars, one of the groups, Lions of Liberty, told local TV station AZFamily.

Yavapai county sheriff David Rhodes issued a statement about drop box watching and voter intimidation this week, saying that the number of ballots a person drops off does not indicate a crime or suspicion of a crime. Arizona’s ballot collection law doesn’t specify how many ballots a person can drop off, just the people they can carry ballots for.

“It is difficult to know each voter’s circumstance so your behavior towards others attempting to cast ballots must not interfere with that person’s right to vote. Should your actions construe harassment or intimidation you may be breaking Arizona’s voter intimidation laws,” Rhodes wrote.

Election officials ask voters to report instances of harassment and intimidation to their local election offices or other authorities, so that those claims can be investigated.