Charles Schwab & Co. said on Tuesday that high trading volumes and a technical issue with a key vendor triggered temporary outages on its trading platform on Monday, a day when its customers rushed to navigate an outsize market sell-off.

A spokesperson for the firm did not identify what kind of technical issue hampered the ability of Schwab clients to log on to their accounts, or name the vendor.

“A combination of higher volumes and a technical issue with a key vendor affecting our systems led to log-on issues and call wait times that were longer than we expect,” the spokesperson said in an emailed statement, adding the issues were resolved and “platforms are fully available.”

Other firms also had issues.

Some U.S. customers reported struggling to log in to their accounts at Fidelity and Vanguard on Monday morning, the firms said in social media posts that day, even as Wall Street’s main indexes opened sharply lower thanks to weak economic data, underwhelming technology firm earnings reports, and increased anxiety about the geopolitical outlook.

Outage tracking website Downdetector reported that nearly 14,500 Schwab users could not access the site, while more than 3,600 users reported problems with Fidelity.

Fidelity confirmed that some of its customers experienced intermittent outages on Monday morning but declined additional comment. A spokesperson for Vanguard declined comment on the cause of access issues on Monday at that firm.