It's a typical Union Square story — a 16-year-old girl and her mom park in the Ellis-O'Farrell garage while they shop for a special outfit, in this case duds for the teen's upcoming cello recital. But when they get back to the car, the story takes a rotten turn: thieves have smashed out the back of their vehicle's window, and have stolen the girl's expensive instrument.

That's what happened to San Ramon teen Serena (her last name has been withheld), who had her $8500 cello swiped from her mom's SUV on Saturday April 11 at around 9 p.m.

"The only thing in my head was, 'How could anyone do this?'" Serena told ABC7. "A cello is obscure and large. What would you do with it?"

"How could anyone do this to anybody?" she asked KTVU.

But it did happen, and now police are seeking the public's help to find the thieves. According to San Francisco Police Department spokesperson Officer Albie Esparza, a male and female team of crooks were caught on video and in stills before and during the crime, making them easy to identify.

"In the video you see the female suspect rummaging through the victim's vehicle while her male accomplice awaits in their getaway car, acting as a lookout." Esparza says.

Here it is, see for yourself:

The video makes it "very evident that they were driving around looking to find anything in any vehicle," SFPD spokesperson Officer Carlos Manfredi told KTVU.

Earlier photos like the one above and below show the suspects waiting for the elevator at the same parking lot where the theft occurred, the Ellis-O'Farrell garage at 123 Ellis Street.

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The instrument, which Serena has been playing with the Berkeley Youth Symphony, is a 2014 Montagnana model Emilla Stradella cello, with a serial number of 1115. It was made by Roland Feller Violin Makers, in SF.

It's a rare enough instrument that it won't be easy to pawn (as a cello thief who stole a similar instrument from the SF Conservatory of Music in 2012 discovered), which worries Serena and her mom.

"We need the cello back, we won't ask questions," Serena's mother told ABC7.

"It's so important to me," Serena told KTVU. "It's so unique that I could never find it anywhere else, anything with the same rich sound and that gives me so much confidence. I just want it back."

According to Esparza, police are "currently following up on several leads regarding the theft" but still need your help to find the thieves. He says that "the first suspect is described as a white male approximately 5’09” 170 lbs with a shaved head, between 30 to 40 years of age."

"The second suspect is described as a white female approximately 5’05” medium build 160 lbs with brown hair, between 20 to 30 years of age." And of course there are the above pictures and video to help you identify them.

Do these folks look familiar? Then please call Sergeant Alicia Castillo of the Tenderloin Station Investigation Team at 415-345-7300, or SFPD's Anonymous Tip Line at 415-575-4444.

Related: Student's Stolen Cello Found!

Still from security footage of couple that allegedly stole a cello from a Union Square parking garage on April 11: SFPD