SAN FRANCISCO — For the last two years, the normally dapper Jack Dorsey has hardly been seen at the offices of Twitter, the social networking company he co-founded, even though he works a block away. So he shocked many when he returned on Thursday with a beard so wild that his own mother made fun of it on Twitter.
“He looked like a mountain man back from the wilderness,” not the incoming leader of one of the world’s largest social networks, said one person who has worked closely with Mr. Dorsey in the past but spoke on the condition of anonymity because of the unusual circumstances of his return.
Mr. Dorsey, 38, is a figure of legend to newer Twitter employees, and remembered with varying degrees of fondness and fear by those who worked with him during his earlier stints there.
Twitter unexpectedly announced on Thursday that Mr. Dorsey would become its interim chief executive while the company’s board searched for a replacement for Dick Costolo, who will step down on July 1.
As the man who sent the first tweet in 2006 and a product visionary who led the company in its early years, Mr. Dorsey helped make the micromessaging service into a global platform that now has more than 300 million active users, from celebrities like President Obama and Katy Perry whose tweets are followed by millions to ordinary people with just a few dozen followers.
But Mr. Dorsey was also a major player in the executive turmoil that has plagued the company throughout its existence, most recently in 2011 when he briefly oversaw product development after he helped install Mr. Costolo as chief executive in a boardroom coup that ousted another co-founder, Evan Williams, from the job.
Mr. Dorsey was the driving force for many of Twitter’s product innovations during that time, like the ability to embed tweets on other sites, but was also a polarizing figure, firing product managers and fostering an atmosphere of secrecy and paranoia, according to current and former associates.
Mr. Costolo, a former entrepreneur and stand-up comedian, has strived to create a more open, collaborative culture at the company. He has built up Twitter’s advertising business, but has largely failed to fix persistent product problems that make it difficult to attract new users. Mr. Costolo has also been criticized for making rapid-fire decisions without a clear strategy guiding them.
It is not clear what role Mr. Dorsey — Twitter’s chairman as well as the chief executive of Square, an e-commerce company — played in Mr. Costolo stepping aside. Mr. Costolo declined to comment on Friday, and Twitter spokesmen did not respond to requests for comment. A spokesman for Twitter’s board declined to comment, as did a spokesman for Square.
Although Mr. Costolo had indicated for months that he was tiring of the job and intended to leave eventually, he had planned to stay until early 2016 to see his turnaround efforts bear fruit and manage the transition process.
At a board meeting last week, that plan changed for reasons that have yet to be explained. Mr. Costolo’s abrupt announcement of his departure was a surprise to many of those at the highest ranks at Twitter, according to two people close to the company.
The general feeling among Twitter employees now is trepidation, according to people inside and outside the company. Many are concerned at the prospect of Mr. Dorsey’s interim title becoming permanent, given his divisive and sometimes erratic management style and the fact that he had been dismissed and returned to the company before.
Wall Street was similarly unimpressed. Shares of Twitter were flat in trading on Friday as investors considered the news.
“Jack is obviously considered a product visionary and could really lead the acceleration of the cadence of product releases,” said Robert Peck, an analyst at SunTrust Robinson Humphrey who had predicted that Mr. Costolo would not last much longer as chief executive. “But the company is in turmoil and to have a C.E.O. who is only part-time involved is a concern.”
Mr. Dorsey, a billionaire who deeply admires the late Apple chief Steven P. Jobs, has not ruled out turning his interim return into a permanent one, much like Mr. Jobs did after his temporary return to Apple in 1997.
“It’s not something I’m thinking about or focused on,” Mr. Dorsey said in a brief interview on Thursday. “My entire focus is to make sure that Twitter is going to be amazingly successful.”
Associates say he has in the past discussed merging Twitter and Square, which would give Twitter an instant e-commerce platform, something that the company has struggled to build. However, at its latest round of venture funding in October, Square was valued at $6 billion, which would make it an expensive acquisition for Twitter, and it is not clear if Mr. Dorsey still sees that as an option.
Mr. Dorsey has high-level supporters at Twitter. Peter Fenton, a partner at the venture capital firm Benchmark and a Twitter board member, has long been an advocate for Mr. Dorsey, according to people close to the company.
The mood among those at Square, where Mr. Dorsey is still chief executive, was mixed after the news broke of his return to Twitter. Some Square employees fear that Mr. Dorsey appears to be choosing Twitter over Square, according to employees who requested anonymity because they were not authorized to speak to the news media. That could hurt the way outsiders perceive Square’s health as a business, its valuation and its recruiting prospects.
“He’s smart, so I’m sure he knows what he’s doing, but Square is in a competitive space in a market that’s going through a ton of changes,” said James Wester, a payments analyst at IDC, a research firm. “I would think now would be the time for Square to have a pretty firm hand on the tiller if it’s going to continue to be competitive.”
At the same time, Mr. Dorsey’s is more of a high-level conceptual role that is not deeply felt in its day-to-day operations, according to sources close to and inside the company. He is most interested in Square Cash, the company’s consumer-facing money transfer product. Square Cash is gaining in popularity, but it is far from Square’s core business of processing payments and lending money to small and midsize businesses.
Mr. Dorsey tried to assuage any concerns his staff had at an all-hands meeting at Square’s headquarters on Thursday, according to people who were present. He emphasized that he was committed to the company and that Square’s strong leadership team afforded him the opportunity to take the interim chief position at Twitter.
But it is also unclear when Square will regain Mr. Dorsey’s undivided attention.
“The search committee expects to retain a search firm shortly and to begin a thorough process to identify a new C.E.O.,” Mr. Dorsey told Twitter investors in a conference call. “There is no timetable. The committee will take the time it needs to evaluate candidates, both inside the company and outside.”