Robinhood, one of the largest online brokerages, has grappled with an extraordinarily high volume of trading this week as individual investors have piled into stocks like GameStop. That activity has put a strain on Robinhood, which has to pay customers who are owed money from trades while posting additional cash to its clearing facility to insulate its trading partners from potential losses.

On Thursday, Robinhood was forced to stop customers from buying a number of stocks like GameStop that were heavily traded this week. To continue operating, it drew on a line of credit from six banks amounting to between $500 million and $600 million to meet higher margin, or lending, requirements from its central clearing facility for stock trades, known as the Depository Trust & Clearing Corporation.

Robinhood still needed more cash quickly to ensure that it didn’t have to place further limits on customer trading, said two people briefed on the situation who insisted on remaining anonymous because the negotiations were confidential.