A so-called flash crash in European markets on Monday prompted several indexes to tumble sharply, sparking alarm among investors on a day when trading was thin due to public holidays around the world.
Trading was temporarily halted in several markets just before 9 a.m. London time on Monday after some European stocks abruptly turned lower.
Nordic stocks were hit the hardest, with Sweden's Stockholm OMX 30 share index falling by as much as 8% at one point, before paring much of those losses to close the session down 1.9%.
Other European markets also plummeted for a brief period.
U.S. banking giant Citigroup on Monday took responsibility for the flash crash.
"On Monday, one of our traders made an error when inputting a transaction. Within minutes, we identified the error and corrected it," a spokesperson for Citi told CNBC.
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European markets closed Monday's session sharply lower as investors reacted to the flash crash and digested weak economic data out of China and Germany.
The pan-European Stoxx 600 traded marginally lower Tuesday afternoon as market participants monitored key interest rate decisions worldwide.
What is a flash crash?
A flash crash refers to an extremely sharp fall in the price of an asset followed by a swift recovery within the same day.
They typically take place over a few minutes and are often caused by a trading mistake or a so-called fat finger error — when someone presses the wrong computer key to input data.
High-frequency trading firms have been blamed for a number of flash crashes over recent years.
In January 2020, high-frequency futures trader Navinder Singh Sarao was sentenced to one year of home detention for helping to trigger a brief $1 trillion stock market crash a decade earlier.
Sarao was charged by the U.S. Justice Department, accused of wire fraud, commodities fraud and manipulation, as well as a count of "spoofing" — when a trader places thousands of buy offers with the intent of immediately canceling or changing them before execution.
The fabrication of sudden market activity created a momentum in price that Sarao was able to profit from.
The U.S. made the practice of "spoofing" a crime in 2010 in an effort to tighten regulations following the 2008 financial crisis.
Correction: This story has been updated to reflect that several markets were halted at 9 a.m. London time on Monday.