SAN FRANCISCO (Reuters) - Tesla's TSLA.O stock and bond prices dropped on Wednesday after Chief Executive Elon Musk renewed an attack on a British caver whom he had previously insulted on social media and a day after Mercedes unveiled a challenge to the electric car maker.

The stock dropped 2.8 percent to $280.74, the lowest level since late May, following a 4.2 percent decline on Tuesday after Mercedes introduced its first fully electric car in Stockholm.

The $1.8 billion high-yield bond Tesla issued a year ago hit a record low price on Wednesday. It also became more expensive to insure Tesla’s bonds against default.

Tesla’s stock has lost a quarter of its value since Aug. 7, when Musk tweeted that he had secured funding for a previously undisclosed plan to take Tesla private.

Musk on Aug. 24 abandoned that plan, but he faces lawsuits and a U.S. Securities and Exchange Commission investigation into the factual accuracy of his tweet that funding for the deal was “secured.” The incident is weighing on investor confidence and has hurt the credibility of Musk, long viewed by Tesla supporters as its most valuable asset.

Also raising concerns about Musk’s leadership and focus as Tesla struggles to increase production of its Model 3 mass market sedan, BuzzFeed News reported late on Tuesday that Musk, in an email to the news site, called British cave diver Vernon Unsworth a “child rapist.”

Musk in July apologized to Unsworth for similar insults he made on Twitter following the rescue of a dozen Thai schoolboys and their football coach from a cave in Thailand. Unsworth, who played a leading role in the rescue, drew Musk’s wrath when he said that a mini-submarine created by Musk’s rocket company SpaceX to help in the rescue “had absolutely no chance of working.”

BuzzFeed reported that Unsworth, through his attorney, denied Musk’s accusations, and that it found no evidence to substantiate Musk’s claim. Tesla did not respond to requests from Reuters for comment. Reuters was unable to verify the emails reported in the BuzzFeed article.

“Elon Musk’s ongoing campaign of publishing vile and false accusations against Mr. Unsworth is outrageous,” Unsworth’s lawyer, L. Lin Wood, said in an email to Reuters.

FILE PHOTO: A Tesla sales and service center is shown in Costa Mesa, California, U.S., June 28, 2018. REUTERS/Mike Blake/File Photo

“Musk has publicly and clearly stated that he ‘hopes’ to be sued. Let me be equally clear in response - Musk will be sued - not because of his hopes, but because he deserves to be sued,” Wood said.

Previous attacks by Musk against Unsworth on Twitter, and his surprise announcement about taking Tesla private, have led to concerns about Musk’s judgment and the ability of Tesla’s board to influence his behavior.

“He should not tweet about things outside of car production and the auto industry,” said Tigress Financial Partners analyst Ivan Feinseth, who has a “neutral” rating on Tesla. “The board, even though it is friendly to him, has now been put in a position where the internal governance of the company has been questioned.”

Money-losing Tesla has been burning through cash as it has aggressively ramped up production of the Model 3, a process Musk has called “production hell.” Tesla bulls are counting on Musk to exponentially raise production, leading to long-term profits.

But the company has repeatedly missed production targets.

Marking the start of a German onslaught against the U.S. car maker, Mercedes on Tuesday showcased a SUV with a 450-kilometer (280-mile) range - as measured by the New European Driving Cycle test system - that it hopes will appeal to luxury customers and tech-savvy millennials alike.

A Tesla spokeswoman said Tesla’s Model X SUV, as measured by the European system, has a range of 565 km (351 miles). Under stricter standards used by the United States Environmental Protection Agency, the Tesla’s range is measured at 295 miles (475 km).

Falling prices for Tesla's bonds on Wednesday showed deepening concerns about the company's dwindling cash pile. Its $1.8 billion junk bond maturing in August 2025 88160RAE1= dropped more than a point in price to a record low 85.75 cents on the dollar.

Tesla is rated “B-minus“ by Standard & Poor’s and “B3” by Moody’s Investors Service, six notches into junk bond territory and just one notch above the “C” area typically assigned to distressed issuers. The outlook from both agencies is negative.

The cost to insure Tesla debt against default has been rising, and its five-year credit default swaps TSLA5YUSAX=MP hit a one-month high, according to Markit. Combined with an upfront payment of 17.2 percent of the bond’s insured value, it would cost an investor around $230,000 to insure $1 million of Tesla debt for a year, up from less than $200,000 two weeks ago, when Musk was still exploring a possible purchase of the company.

(The story corrects paragraph 15 to show Mercedes cited a different vehicle range testing standard than the one used by Tesla.)