Every Sunday afternoon since early June, the Rev. Elea Kemler and a dozen or more congregants have held vigil for Black lives on the grassy common in front of First Parish Church. They wave at the stream of cars rumbling down historic Main Street and hoist homemade Black Lives Matter posters.

Some passersby are “wildly enthusiastic,” Kemler said, beeping their horns and waving. A few others are decidedly less so. They scream “Trump!” out their windows and yell obscenities at the parishioners.

“They say, ‘All lives matter,’ as if that’s not the very point,” said Kemler, the church’s minister for the past 20 years. “And they just yell Trump’s name. I don’t know what that’s supposed to be code for.”

Like many cities and towns jolted by the tragic deaths of George Floyd, Breonna Taylor, and other Black Americans, Groton — a bucolic suburb of some 11,300 people — is trying to reconcile an ugly past with its present image. Today the lawns along Main Street are dotted with Biden-Harris yard signs. A giant rainbow flag hangs beneath the belfry at First Parish. Each of the town’s entrances is adorned with an engraved stone perched on the side of the road proclaiming “All are welcome.”

Yet the town remains more than 90 percent white, which, like many American suburbs, may not be an accident. Groton was once a Ku Klux Klan stronghold, rife with anti-Catholic and nativist prejudice. In an online database created by author and sociologist James Loewen, Groton is listed as one of 17 possible, though unconfirmed, “sundown towns" that once existed in Massachusetts. Sundown towns, found in states across the country, were all-white jurisdictions that, for decades, excluded or expelled religious and ethnic minorities, usually Black people, sometimes by law and more often through violence and intimidation. In some of these places, signs posted at the city limits ominously warned Black people that they weren’t welcome after dark.

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Loewen, the sociologist and author of “Sundown Towns: A Hidden Dimension of American Racism," wasn’t surprised no written record exists to confirm Groton’s status as a sundown town.

“Since this is a shameful topic, oral histories are better,” Loewen said. In sundown towns, according to Loewen, ordinances explicitly forbidding religious minorities and people of color from renting or owning property were far less common than informal policies — enforced by police, vigilantes, realtors, and banks — that dictated who could stay and who would be forcibly driven out.