Klara and Johanna Soderberg stood in front of the silent synagogue and hummed in precise unison. Backed by only a light, tight guitar strum, they deftly covered Simon and Garfunkel’s “America,” an ode to discovering the country through vinyl, interstates and open pastures. The sisters have an infatuation with Americana and all its folksy, free-spirited splendor and, extraordinarily, they do it justice.

The Soderberg sisters, better known as First Aid Kit , grew up in the suburbs of Stockholm, rose to YouTube fame in 2008 and by 2010 were touring the world and feasting on American folk rock, from Patti Smith to Bright Eyes. They recruited the latter to help produce their 2012 album, “The Lion’s Roar.”

When First Aid Kit finally performed covers of “America” and Smith’s “Dancing Barefoot” for the artists themselves at the ceremony for Sweden’s Polar Music Prize, they brought Paul Simon to his feet and Smith to tears.

“I think it went pretty well,” Johanna, 21, told the sold-out crowd at Sixth & I Historic Synagogue on Wednesday, about that experience. “Yeah, I think it went pretty well,” Klara, 19, echoed.

First Aid Kit didn’t officially form until 2007, but the sisters’ father, a guitarist in the 1980s Swedish rock band Lolita Pop, had been grooming their vocals from a young age. Somewhere along the way, an obsession with Woodstock-era Americana set in. The duo’s latest single, the country-leaning “Emmylou,” is a twangy tribute to June Carter, Johnny Cash, Gram Parsons and Emmylou Harris. And the chord progressions in “Our Own Pretty Ways,” a track from their 2008 EP “Drunken Trees,” have hints of Simon’s “Graceland.”

Klara Soderberg of First Aid Kit, half of the sisterly duo from Sweden, performs a sold-out show at Sixth & I Historic Synagogue on Tuesday night. (Josh Sisk/FOR THE WASHINGTON POST)

On Wednesday, they took the stage draped in sheer paisley dresses and with hair grown to their waists, looking more like Topanga Canyon flower children than refined, world-class vocalists. Yet the Soderbergs howled, moaned and bellowed in perfect harmony, weaving one big, colorful blanket of sound. What’s more, they made the whole thing look easy when it most certainly was not.

They powered through such mighty ballads as “Lion’s Roar” and tiptoed through the more subtle songs, such as ”Marianne’s Son.” For “Ghost Town,” a catchy and tender song about rejecting religion, they stepped away from the microphones to the edge of the stage and invited the crowd to join them as they, ironically, belted into the cavernous temple:

“If you’ve got visions of the past, let them follow you down / They’ll come back to you someday. / And I found myself attached to this railroad track / But I’ll come back to you someday.”

There were moments when the image and sound seemed contrived, but why wouldn’t it be possible that these girls really are that earnest? And with this level of raw talent, why does it matter? There are worse things than a pair of modern-day Joni Mitchells.