If you think voter fraud is an unprosecuted crime, tell that to the Maryland investigators who knocked on my front door in 2014.

Here’s how I learned that someone voted as me in the 2012 general election.

On March 26, 2014, three investigators from Maryland’s Office of the State Prosecutor sat at my dining room table and showed me a signature on a photocopy taken from a D.C. poll book. The scrawl looked more like a seismograph reading and was so unrecognizable that it took me a minute to realize that I was looking at it upside down. Turning the picture over didn’t make it much better.

“No, that’s not my handwriting,” I told them.

Somebody had clearly voted using my name. But why? And how did state officials figure it out?

In-person voter impersonation is vanishingly rare, as many studies have shown. The claims put forth by Donald Trump that voter fraud in places like Philadelphia could rig the election against him have very little evidence behind them, according to election experts.

Absentee ballot fraud – people violating state laws on the distribution, collection and submission of mail ballots – is the more likely and more commonly prosecuted crime. Even for these kinds of scams, a definitive total of cases is hard to come by since voting records are maintained by several thousand different local governments.

In Montgomery County, Maryland, where I live, a woman named Linda Earlette Wells cast a provisional ballot in the name of her dead mother in the 2012 election (Wells was a registered Florida voter at the time). After doing so, she called the county’s board of elections and told an employee that she had used her mother’s name and ID to vote and wanted to cancel the ballot. By pleading guilty, Wells was sentenced to probation without a conviction appearing on her record. Hers was one of two cases of voter fraud brought by Maryland authorities since 2012.

I voted early in 2012, because at the time I worked at The New York Times and was expected to be in the newsroom in Manhattan on Election Day. When the investigators came to visit me in 2014, the picture of the poll book they showed me was from a polling place I hadn’t voted in for eight years, having long since moved out of the city and into the nearby Maryland suburbs. Somehow, my name had remained on the D.C. voter rolls, and someone had apparently taken advantage of it.

Voting in someone else’s name is difficult to arrange. You’d need to know that the person you were attempting to vote as was not going to show up themselves, and you’d need that person to live in a jurisdiction where the vote would help elect or defeat someone. D.C. isn’t known for having competitive races, and in 2012 there was one citywide contest – an at-large city council seat – that was less than a blowout. I struggled to think why voting as me would be worth doing, and also how it was possible to do it.

When you move from one state to another (or, in my case, from the District to Maryland), registering to vote in your new jurisdiction creates a new record there, but doesn’t always cancel the old one.

The District does have a process for removing voters who do not vote once every four years from the rolls, but that clearly didn’t happen in my case. Without a national election authority, it’s up to states to find and investigate issues of voter fraud. One way to make that job easier is to keep accurate voter lists, and tools such as online voter registration systems can help. An accurate voter list makes it less likely that mistakes will occur at the polls.

In my case, I registered in Maryland when I got my driver’s license, an option encouraged by the Motor Voter Act, but since Maryland and the District separately maintain voter lists, there was no immediate trigger that caused my D.C. voter registration to be cancelled. I didn’t check, either.

So how did the Maryland investigators know that someone claiming to be Derek Willis appeared to vote in both Maryland and D.C. in 2012? And why did it take them so long to show up at my door?

After each election, local election authorities produce what are known as “voter history” files that show, for a given election, which registered voters actually voted (but not how they voted, of course). These files are public records in most states. Political parties and campaigns often pay vendors to maintain voter files to help them identify and target voters, but members of the public and government officials can also access voter history in most places. In my case, it’s likely that Maryland officials compared their own voter history files to D.C.’s, looking for people with the same name and date of birth or age who appeared on both.

That process doesn’t guarantee that people identified as voting in both jurisdictions are, in fact, the same person. Journalists are familiar with the pitfalls of this kind of “fuzzy matching” based on names and ages. And though we don’t share a birthday or a jump shot, I get mistaken for University of Kentucky basketball forward Derek Willis often enough on social media that I was once invited on a Kentucky radio show to talk about it.

There is no federal government agency that maintains a comprehensive set of voter registration and history records. It’s unlikely that the federal government could even compel states to participate in one; the constitution hands the power to run elections squarely to the states. However, some 20 states and the District of Columbia participate in a data-sharing project called the Electronic Registration Information Center.

The ability to match voter information across state lines is one reason the Maryland investigators showed up at my doorstep. They asked me where I was on Nov. 6, 2012, and whether I had voted in D.C. that day. Once I explained where I had been on Election Day (and was able to provide some evidence of it), the investigators’ interest diminished. When I called the office this fall to find out what, if anything, happened, I was told the case was dropped, and since there was no way to tell who had voted in my name in D.C. that day, there were no other leads to follow.

To be sure, I don’t know if whoever signed my name did so with fraud in mind. He or she could have simply signed in the wrong place in the poll book. It would have been a particularly frustrating crime. Even that “close” at-large city council race was won by nearly 70,000 votes.

This election year, some people claim voter fraud goes under-reported and unprosecuted. The three investigators from the Maryland Office of the State Prosecutor who sat at my dining room table would surely disagree, and a listing of voter fraud cases compiled by the Heritage Foundation from public reports includes instances from dozens of states that involve prison sentences for the perpetrators, albeit involving different kinds of crimes.

After the investigators left my house that day, the first thing I did was call up the D.C. Board of Elections web site to check my voter registration record. By that time, they had finally gotten me off their rolls.