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The stock-trading app Robinhood suffered an outage Monday for the third time in a week, frustrating users of the zero-fee tech startup who tried to place trades during the day’s market plunge.

“Trading is currently down on Robinhood, and we’re investigating the issue,” the company said on one of its verified Twitter feeds. “We’re focused on getting back up and running as soon as possible.” By 10:25 a.m. ET, most trading was functional again for new orders, it said.

The Silicon Valley startup has upended Wall Street in recent years by offering trades of stocks and bonds without charging commission, undercutting traditional rivals such as Charles Schwab, which have since followed suit with their own zero-fee trades.

The app combines a slick, confetti-filled interface with an ability to trade riskier investments including options and cryptocurrencies, worrying personal finance experts who say Robinhood may fuel excess risk especially among those new to investing.

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But Robinhood has had difficulty just keeping its service online during the financial turmoil tied to the coronavirus fears. Traders who said they were unable to place orders for two days last week lashed out at the company on Reddit and Twitter, where one account advocating for a class-action lawsuit has attracted thousands of followers.

“Surely this can’t be legal. Constant broker outages on the biggest trading days in history? No way,” one Reddit user wrote Monday.

The company’s founders blamed the crashes last week on an “unprecedented load” of trading volume and account sign-ups, saying in a blog post that they were working to reduce the interdependencies in their infrastructure and investing in additional redundancies.

The latest outage affected trading in equities, options and cryptocurrency, Robinhood said on a corporate site. The company, based in Menlo Park, California, said it was providing further information to customers via email.

Late on Monday, Robinhood said in an additional statement that it believed the latest interruption wasn’t related to the outage from last week and that the company was trying to understand the cause. The service was fully operational by 3:30 p.m. ET Monday, it said.

“We know this interruption was frustrating for our customers — especially after last week and on a day that trading was halted marketwide,” the company said.

The Financial Industry Regulatory Authority (FINRA), which oversees stock brokers, said it was in contact with Robinhood and monitoring the situation. The authority declined to comment Monday whether it was opening an investigation.

Major Wall Street incides plummeted Monday more than 6 percent as a fight over crude oil production created heightened pressure on a global economy already suffering the effects of the coronavirus epidemic.