"This past year has been the most difficult and painful year of my career," Musk told the New York Times in an interview Thursday. "It was excruciating."

The paper said the electric car company's swashbuckling boss choked up at times during the interview as he talked about working marathon hours trying to meet production deadlines.

Musk, who founded Tesla in 2003 to push cars into a new tech era, said he saw last week's tweet -- which included an assurance that funding for going private was secured -- as an attempt to be transparent.

The news sent Tesla shares soaring so much that trading was suspended in New York.

It shocked investors, market analysts and reportedly even Tesla board members who did not see it coming, as well as attracting the attention of regulators.

Tesla shares fell this week on reports by Fox Business and the New York Times that the US Securities and Exchange Commission has subpoenaed Musk to talk about the tweet.

Normally such a major announcement -- taking a huge company private -- would be explained in detail beforehand to regulators.

He and other board members are preparing to meet with SEC officials as early as next week, the Times said.

Musk acknowledged that no one read his Twitter post before he sent it, but said he did not regret it. "Why would I?" he said.

- Board fears -

Musk, who envisions sending tourists to the Moon, is recognized as one of the most influential innovators in the United States.

California-based Tesla has become one of the most valuable automakers on expectations it will disrupt the industry.

But the South African-born Musk has appeared to fray under the pressure in recent months.

Tesla produced only slightly more than 100,000 vehicles last year and the company has been struggling to boost production of its Model 3, which is less expensive than its first models and could help expand Tesla's base.

On the physical toll his job is taking, Musk said: "It's not been great, actually. I've had friends come by who are really concerned."

He described spending every hour of his 47th birthday in June at work and almost missing his brother's wedding.

On Monday, the Tesla boss explained in a blog post that his much scrutinized statements about financing were based on his conversations with Saudi Arabia's sovereign wealth fund and other investors.

Taken unawares, the firm's board said this week that it has formed a special committee to consider going private.

If delisted, Tesla could operate without requirements for financial reports and other pressures of a publicly traded firm. But it could also lose visibility and limit its ability to raise capital.

Musk told the Times that board members had not complained to him about his tweet.

But the paper said he later told them through a spokesperson that Antonio Gracias, Tesla's lead independent director, had in fact contacted him to discuss the Twitter post.

Musk added he had agreed not to tweet again about taking the firm private unless he had discussed it with the board.

The tycoon has caused controversy by turning to his Twitter following of more than 22 million to vent frustrations in recent months.

In one bizarre moment in July, Musk labeled a British caver a "pedo" -- or pedophile -- after the rescuer dismissed the Tesla chief's idea for bringing 12 trapped Thai boys to safety in a miniature submarine.

The Times report said board members had raised concerns over his "workload but also about his use of Ambien", a sleeping drug.

Musk responded to speculation that his comment that he wanted to launch the buyout at $420 a share was an oblique reference to cannabis counterculture.

"But I was not on weed, to be clear. Weed is not helpful for productivity," he told the Times.

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TESLA MOTORS

THE NEW YORK TIMES COMPANY