By Michael Liedtke, Technology Writer

SAN FRANCISCO (AP) --

Four of Twitter's key executives are leaving the company in an exodus that has escalated the uncertainty facing the messaging service as it struggles to broaden its audience and lure back disillusioned investors.

Twitter CEO Jack Dorsey announced the management shake-up late Sunday after technology news site Re/Code reported the changes earlier in the day.

Dorsey described the departures as voluntary, a characterization that three of the four exiting executives echoed in their own posts.

The upheaval leaves Twitter without its top engineering executive, Alex Roetter; its top products executive, Kevin Weil; its head of human resources, Skip Schipper; and Katie Stanton, who oversaw the company's media partnerships.

Dorsey is turning over supervision of the human resources and media teams to Twitter's chief operating officer, Adam Bain, and assigned engineering to the company's chief technology officer, Adam Messinger. Bain and Messinger will share some of the responsibilities for creating and running Twitter's various products.

This is the second major fissure in Twitter's ranks since Dorsey was named the San Francisco company's permanent CEO in October. His hiring followed a three-month stint as interim CEO after the resignation of his predecessor, Dick Costolo.

In one of his first moves, Dorsey laid off more than 300 employees, or about 8 percent of Twitter's workforce, to trim expenses at a company that has never turned a profit since its service started nearly a decade ago.

It's not unusual for CEOs to reshuffle management shortly after their arrival.

But analysts interpreted Twitter's changes as a distress signal instead of a reason to hope that the company is starting to head in the right direction.

"We don't see how the departure of the heads of three major business divisions can be viewed as a positive in the middle of an attempted business turnaround," Stifel analyst Scott Devitt wrote in a research note. Devitt had been recommending Twitter's stock before the management departures prompted him to reverse his opinion.

In another note, Citi analyst Mark May said the defections may prolong Dorsey's attempt to turn around Twitter and could hint at problems that may surface Feb. 10 when the company is scheduled to report its results for the final three months of last year.

Twitter's stock shed 56 cents, or 3 percent, to $17.28 in Monday's afternoon trading. The shares have plunged by 52 percent since Dorsey became interim CEO last July, leaving them well below their price of $26 when they were sold in an initial public offering in November 2013.

The downturn reflects concerns that Twitter isn't going to get much bigger than it is now, after a long streak of torrid growth in its early years. Twitter's active users grew from 308 million in March to 320 million in September, a measly gain compared to the much larger Facebook, which added 104 million users during the same span. Facebook has more than 1.5 billion active users.

Twitter is trying to make its service more appealing with a feature called "Moments" that compiles photos, video and messages about big news events. Twitter also is considering expanding the restrictions on tweets beyond the 140-character limit that has defined the service since its debut.

Now, Twitter's board appears ready to undergo a makeover that Dorsey has been hinting would happen.

Citing unidentified people familiar with the matter, both The Wall Street Journal and The New York Times reported Twitter is about to name two new directors, including a well-known media executive, to its eight-member board. It's not clear which of Twitter's current directors will leave.

Twitter last year named a former Google executive, Omid Kordestani, as its executive chairman.

Although the changes will probably increase the turmoil within Twitter, Gartner Inc. analyst Brian Blau believes they're necessary.

"Losing a pretty significant chunk of their management team should open the door to even more changes," Blau said. "This is a signal that they are planning to do something different, and that could be good. It's pretty obvious that something is not working there."

MySHOOT Profiles