A single trade on a sell order caused the sudden drop in European shares and a crash in Nordic markets. Citigroup IncPeople who are familiar with this matter spoke to Reuters.

Flash-crash caused European shares to fall suddenly on holiday-thinned trading days. This was due to an error in calculation. Nasdaq IncAccording to one source, indexes could include Swedish companies

After a 5-minute plunge, the Stockholm OMX30 equity benchmark index fell by 8%, but recovered most of its losses and traded down 1.7% at 1229 GMT.

Citi refused to comment.

Nasdaq had previously said that the sudden drop in stock prices was caused by a single sale-side order.

“The cause of the drop was caused by a sell event from a market participant. We have not identified any disturbances in Nasdaq´s systems,” a Nasdaq spokesperson in Stockholm said in an emailed statement shortly before market close.

“Furthermore,” she stated, “Nadaq, following a review has not seen any reason why to cancel trades which were made during the event.”

Brokers said the flash crash was caused by an incorrect trade that occurred on a day of low activity.

The Swedish financial supervision authority stated earlier that it was looking into the unexplained plunge. It was also in touch with Nasdaq in the United States, who runs Stockholm and the other Nordic stock markets. Nasdaq quickly ruled out any error in their systems.

Other indexes fell as well, including the ones in France, Germany, Norway, Germany and Italy, which later rebounded.

This outsized movement extended the pan-European reach STOXX 600The equity benchmark plunged by more than 2 percent in a matter of two minutes, starting around 7:58 GMT. But the London markets closed for holidays and volume was reduced.

An indicator of volatility in euro zone stocks saw an abrupt spike at its highest point since mid March, 35.99.

Martin Munk, Jyske Bank’s equity sales vice-president for equity sales, stated that it was “weird” in the minutes following the dip. He also said that worried clients called him to inquire what was going on.

Munk claimed that the trade appeared to be an error, or a technical glitch. There was no news which could have caused it.

Nordnet broker earlier that day spoke about a market “flash” which caused brief panic. London traders, however, suggested it was caused either by an algorithm-based trade going haywire (or a huge “fat finger”) trade.

It was just days before an anticipated interest rate increase by the U.S. Federal Reserve. This is amid concerns that too aggressive tightening of policy could lead to a slowdown in the global economy, already suffering from high inflation.

Nasdaq stated that it constantly monitors the price movement on its market and is in dialogue with market participants about Monday’s volatility.

A spokesperson for Nasdaq stated in an email that “we currently see nothing to indicate error in Nasdaq’s own systems.”