'Sherlock': Benedict Cumberbatch, Martin Freeman brood in Season 4 photo

LONDON, Aug. 31 (UPI) -- Popular BBC series Sherlock is gearing up for Season 4.

The show's official Facebook page released a photo Wednesday of Benedict Cumberbatch and Martin Freeman as a brooding Sherlock Holmes and John Watson.

"The boys will be back soon! Here's the first official picture of #Sherlock and John from Series 4," the caption reads.

Season 4 completed filming this month and is expected to premiere in 2017. The season will feature the return of Holmes' presumed-dead nemesis Jim Moriarty (Andrew Scott) and will reportedly be darker than previous seasons.

"It's really dark," Amanda Abbington, who plays Mary Morstan, said at San Diego Comic-Con in July. "It's the darkest [executive producers] Steven [Moffat] and Mark [Gatiss] have written ... If we can pull this off, then it's amazing."

Sherlock released a first trailer for Season 4 the same month. The show is expected to continue through Season 5, although Moffat recently had doubts when he spoke to the Telegraph.

"I don't know how long we can keep going," he admitted. "I'm personally willing but I'm hardly the main draw. I would be moderately surprised if [Season 4] was the last time we ever made this show. But it absolutely could be."

Benedict Cumberbatch (L) and Martin Freeman as Sherlock Holmes and John Watson for "Sherlock." Photo by Sherlock/Facebook