Bloomberg Television

Mark S. Mahaney has been a stock-picking star among a wave of new technology companies, a number-cruncher with Silicon Valley flair.

But his career took a bizarre turn on Friday, as he was swept into a regulatory investigation of Wall Street that stems from Facebook’s messy stock debut.

Citigroup fired Mr. Mahaney early Friday morning, after regulators discovered that he answered a reporter’s e-mail without the bank’s blessing. When confronted by the bank about the brief exchange, which was unrelated to Facebook, he covered up his mistake.

The revelations emerged from a broader inquiry into Wall Street’s role in Facebook’s initial public offering. The Securities and Exchange Commission and the Massachusetts secretary of the commonwealth, William F. Galvin, have subpoenaed Citigroup and others like Morgan Stanley over the $16 billion offering. The authorities are examining whether bank analysts violated a cornerstone of the securities industry when they shared research about Facebook with a limited group of clients and journalists.

In some ways, the investigations echo the crackdown more than a decade ago on the practices of analysts who covered dot-com companies.

“This is an especially sensitive area due to the research scandals of the Internet bubble and the fact that the Facebook I.P.O. was so visible,” said Jay Ritter, a finance professor at the University of Florida.

Yet regulators did not accuse Mr. Mahaney of improperly leaking information about Facebook. (His disclosure was about YouTube’s earnings.) Still, a junior Citigroup analyst does face such claims. In a civil order released on Friday, Mr. Galvin accused the unnamed analyst of sharing nonpublic information about Facebook to TechCrunch, a blog focused on the technology world. Citigroup, which fired the junior analyst in September, paid a $2 million fine on Friday to settle the accusations.

“This penalty should serve as a warning to the industry as a whole,” Mr. Galvin said. “If we’re going to have a level playing field, you can’t selectively give out information.”

Favoritism and selective disclosures were among the abuses uncovered by regulators after the Internet bubble burst in 2000. Investigations, often citing internal e-mails, found that analysts hyped the stocks of companies that did business with their banks while investors lost millions and millions of dollars. In some cases, analysts privately sneered at stocks they were publicly praising. In one e-mail, an analyst labeled a security a “piece of junk.”

The scandal led to a landmark 2003 settlement with several banks, including Citigroup. The case, led by a former New York attorney general, Eliot Spitzer, built a Chinese wall between Wall Street research analysts and investment bankers. Under the court settlement, analysts were barred from communicating with bankers unless accompanied by a compliance officer, an effort to curb bankers from influencing research.

Among other practices widely adopted on Wall Street a decade ago, analysts had to obtain in-house permission before commenting publicly to news media about companies they covered. Some Wall Street firms also prevent analysts from expressing a viewpoint on companies unless the information is published in a report.

These procedures, regulators say, tripped up Mr. Mahaney.

Mr. Galvin’s order took aim at the longtime analyst for discussing YouTube’s earnings with a reporter from a French magazine, Capital, without Citigroup’s permission. The reporter approached Mr. Mahaney in April seeking projections about YouTube’s revenue and earnings growth, some of which Citigroup had not yet published. Mr. Mahaney sent a terse e-mail reply that answered the essence of the reporter’s questions.

But when a bank spokeswoman followed up to remind Mr. Mahaney about seeking approval beforehand, he denied ever e-mailing the reporter. “I won’t respond,” he said, according to the order.

The reporter later informed Citigroup that Mr. Mahaney did in fact respond. According to Mr. Galvin’s order, the analyst then asked bank employees to fudge the timing of the interview, an apparent effort to avoid blame for not seeking approval.

When told that the accurate time was already submitted, he replied in an e-mail cited in the order: “This could get me into trouble. Shoot.”

Mr. Mahaney, 46, who was based in the bank’s San Francisco office, declined to comment.

In a statement, a Citigroup spokeswoman said the bank was “pleased to have this matter resolved,” adding that “we take our internal policies and procedures very seriously and have taken the appropriate actions.”

After firing the junior analyst in late September, and moving toward a settlement with Mr. Galvin in recent days, Citigroup summoned Mr. Mahaney to New York on Wednesday, people briefed on the matter said. Lawyers outlined their concerns about his behavior, but they sent him back to the West Coast without deciding his fate.

However, after striking the settlement with Mr. Galvin on Thursday, the bank decided to dismiss Mr. Mahaney. Some Citigroup officials said at issue was the cover-up rather than speaking to the news media without permission, which is fairly common on Wall Street. Mr. Mahaney learned of the decision around 5 a.m. Friday, the people said.

In dismissing Mr. Mahaney, the bank lost a marquee analyst who covered the recent run of technology stock offerings. He came to Citigroup in 2005 after stints in government at the State Department and on Wall Street at Morgan Stanley and the Galleon Group, the hedge fund that was toppled by insider trading accusations. (Mr. Mahaney was not implicated in that case.)

A regular on the business TV circuit, he was named the No. 1 Internet analyst for five consecutive years in an influential poll by Institutional Investor magazine. His presence helped Citi win a few Internet I.P.O. assignments in 2011, including Zillow and Active Network, according to top executives at those companies.

In Silicon Valley, Mr. Mahaney also became known for regular meetings of money managers and venture capitalists called “the Rally in the Valley” at a restaurant in Redwood City. Among his best calls was a recommendation of Priceline.com in 2008 before it tripled and an upgrade of Amazon in 2009 before its stock price more than doubled.

Mr. Mahaney, who took home about $3 million in 2011, covered 31 companies over the last 12 months as a Citigroup analyst, according to Bloomberg data.

The bank said Friday that it would abandon some of the companies he covered and transfer others to another analyst.

Despite the focus on Mr. Mahaney, the main legal violations stemmed from the disclosure of Facebook information.

In May, the unnamed junior Citigroup analyst e-mailed two TechCrunch employees to say “I am ramping up coverage of FB and thought you guys might like to see how the street is thinking about it (and our estimates).”

He attached a “Facebook one-pager,” which featured an array of confidential information, including Mr. Mahaney’s revenue estimates meant as an internal guide for the bank’s analysts. The leak, which came before the company’s public shares started trading in May, also included Citigroup’s private projections for the company’s “Investment Risks” and “Investment Positives.”

A TechCrunch employee sought to post the document on the Web, but the junior analyst balked.

“My boss would eat me alive,” he said.