LONDON — There will be more mysteries to solve for the sleuth of Baker Street.

The BBC confirmed Saturday that filming on a fourth series of “Sherlock” will start next spring.

The modern-day update of Arthur Conan Doyle’s classic detective stories stars Benedict Cumberbatch as Sherlock Holmes and Martin Freeman as Dr. Watson.

Since “Sherlock” first aired in 2010, Cumberbatch has become a global star, Academy Award-nominated for “The Imitation Game.” Freeman has starred in “The Hobbit.”

The BBC in Britain and PBS’ “Masterpiece” in the United States have also announced they will broadcast a 90-minute “Sherlock” special episode on Jan. 1. Titled “The Abominable Bride,” it sends the characters back to the stories’ late 19th-century setting.

When it airs, the fourth season will be somewhat darker than the first three, according to the British newspaper Express.

Scottish screenwriter Steven Moffat was quoted as saying, “We’ve got a clear idea of what we’re going to do and it’s going to be a clear shift in tone. Yes, there’s going to be some dark stuff coming but there always is. There always fun and jokes and detection and all that nonsense.”

He added: “It’s dark in some ways – obviously it’s great fun and a Sherlock Holmes romp and all that – but there’s a sense of things coming back to bite you.”