SAN FRANCISCO (REUTERS) - Former securities officials and professors said Tesla chief executive Elon Musk may have missed a key disclosure deadline and filed the wrong paperwork when he bought 9 per cent of Twitter, a platform regularly used by the outspoken billionaire.

US Securities and Exchange Commission (SEC) regulators could use any shortfall to try to punish Mr Musk more for other lapses, some believe.

Mr Musk on Monday disclosed that he bought a 9.2 per cent stake in Twitter, making him the micro-blogging site's largest shareholder and triggering a rise of more than 27 per cent in the company's shares. The filing said that March 14, 2022, is the date of the event that required the disclosure.

United States securities law requires disclosure within 10 days of acquiring 5 per cent of a company, and in Mr Musk's case the 10-day deadline was March 24. A late report could lead to a per-violation civil penalty of up to US$207,183 (S$281,607), according to Georgetown University Law Centre law professor Urska Velikonja.

That is a financial slap on the wrist for Mr Musk, the world's richest person with a net worth of US$302 billion, according to Forbes, but the SEC could look into market manipulation allegations regarding the Twitter stock purchase and seek harsher sanctions in an ongoing investigation regarding his Tesla stock sales, experts say.

"This is not really a gray area. He acquired it and didn't file within 10 days. It's a violation. And so this is a slam-dunk case from the SEC perspective," said University of Michigan Law School law professor Adam C. Pritchard.

In addition, Mr Musk filed a "13G" disclosure form for investors who plan to hold their shares passively, even though it emerged on Tuesday that he will take a Twitter board seat in order to push change at the company.

That means he should have filed the "13D" form used by activist investors, officers and directors who have the ability to influence the management and policies of an issuer, said several lawyers.

Mr Musk on Tuesday amended his earlier filing and filed the 13D form to report a change in his status to an active investor.

The SEC is already investigating Mr Musk's Nov 6, 2021, tweet asking his followers whether he should sell 10 per cent of his Tesla stake.

Mr Musk is also bound by a 2018 SEC settlement that requires him to obtain preapproval on some of his tweets, after he tweeted that he had "funding secured" to take Tesla private. The SEC said that he defrauded investors.

Mr Musk also made comments about Twitter after his purchase but before disclosing his stake.

On March 25, Musk tweeted a poll: "Free speech is essential to a functioning democracy. Do you believe Twitter rigorously adheres to this principle?"

A day later, Mr Musk said he was giving "serious thought" to building a new social media platform.

"Arguably, his social media posts about potential alternatives to Twitter can be seen, in light of his previously undisclosed stake, as a form of market manipulation to affect the share price, but proving that seems difficult," said Howard Fischer, a former SEC counsel and a partner at law firm Moses & Singer.

Twitter shares have surged since mid-March when Mr Musk purchased his stake. The stake, valued at around US$2.4 billion at the closing price of March 14, jumped to US$3.7 billion as of Monday's closing price.

In addition, some well-timed trades in Twitter options days before Mr Musk revealed his purchase are raising eyebrows among options analysts.

The SEC would likely investigate if anyone who knew about the acquisition of the shares traded in advance of the filing, said Jacob Frenkel, a former SEC enforcement attorney and government investigations and securities enforcement practice chair for law firm Dickinson Wright.

"I really think that would be the focus rather than the tardiness," Mr Frenkel said.