The London Stock Exchange (LSE) has said that the trading fault that knocked out dealings on Tuesday was caused by human error that "may have occurred in suspicious circumstances".
Morning trading was suspended on one of the LSE's share trading platforms for almost two hours.
The LSE said it had notified the "relevant authorities" and was investigating.
The fault will delay modernisation of the LSE's main share trading platform.
The knock-out occurred on the LSE's Turquoise pan-European share trading platform.
In a statement, the LSE said that investigations into the fault - which occurred between 0823 GMT and 1030 GMT on Tuesday - had "revealed that human error was to blame for the disruption".
It added: "Preliminary investigations indicate that this human error may have occurred in suspicious circumstances."
Turquoise recently started using a new technology platform, supplied by the LSE-owned software supplier MillenniumIT.
The LSE was planning to start using the system on its main share trading platform next month, but will now delay that move.
The new system makes trading faster - and therefore more attractive to high-frequency trading firms that generate huge volumes.
The glitch is the second at Turquoise in less than a month, after a problem with a network card forced the exchange to shut the system for more than an hour on 5 October.
That problem came just two days after Turquoise had moved across to the new technology platform.