(Bloomberg) -- ION Trading UK, the technology firm hit by a cyberattack last week, told some clients that it has rebuilt relevant systems and will relaunch them after carrying out independent security testing.

ION indicated that some systems could be available on Monday, according to people familiar with the matter. The process of bringing clients back online will be staggered and is likely to take place over several days, said the people, who asked not to be identified discussing private information. Cybersecurity firm CrowdStrike has been involved in testing the systems, two of the people said.

One impacted brokerage, StoneX Financial Ltd., said it expected to be provided with a newly built system on Monday, according to a note to clients seen by Bloomberg.

“As we work to return to full service it may take longer than usual to process some payment and transfer requests due the manual processes involved,” StoneX told clients on Monday. “We appreciate your ongoing patience as we work through solutions and push ION on their remediation plan.”

A representative for StoneX declined to comment.

Reactivating systems

Last week’s attack on ION upended derivatives trading around the world, forcing a number of banks and brokers to process certain trades manually and drawing the attention of government agencies and regulators.

Lockbit, the hacking group behind the attack, said on Friday that a ransom had been paid and that the gang provided a decryption key to unlock the compromised computers. ION has declined to comment on the ransom.

ION has been working with two “template clients” to reactivate a key trade-processing system called XTP Core and it will progress with other clients pending validation from CrowdStrike, ION said in a Feb. 6 client update obtained by Bloomberg. 

Six out of 33 key recovery steps had been fully completed and the remainder were between 79% and 95% complete, ION told clients. 

Like ION, Bloomberg LP, the parent company of Bloomberg News, also provides financial institutions with execution management solutions, connectivity to electronic markets and trading tools.

(Updates with information on StoneX from third paragraph.)

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