LONDON, Jan 21 (Reuters) - Two men suspected by Germany of assisting an Islamist gunman, who killed four people in Vienna in 2020, used the major cryptocurrency exchange Binance, German federal police said in a confidential letter seeking information from the company. Germany"s Federal Criminal Police Office (BKA) said in the March 2021 letter, which was seen by Reuters, there were indications that the suspects bought or sold an unspecified amount of cryptocurrency on Binance. Prosecutors have identified the men as Drilon G., a German national, and Blinor S. of Kosovo. Reuters is also withholding their full names. Blinor S. used a bank account to carry out "several" transactions with Binance, the BKA wrote. A Binance verification code from February was found on Drilon G."s phone, it added. The BKA did not give details of the dates, number, or value of the transactions. It asked Binance to provide data relating to the pair, including all digital currency transactions. The request, it said, was in connection with "potential terrorist attack plans," without providing further detail. Reuters couldn"t determine how Binance, the world"s biggest cryptocurrency exchange by trading volumes, responded to the letter. Binance didn"t comment. The BKA declined to comment. Blinor S. and Drilon G., in messages exchanged with Reuters, denied assisting the gunman, Kujtim Fejzulai, and using cryptocurrencies to finance his or any other attack. Blinor S. said he opened a Binance account in February and used it solely to invest in different cryptocurrencies. "I know that every transaction on Binance is traceable," he said. Lawyers for both men said neither had been formally charged with any crime and no arrest warrants have been issued. Since last year Binance has come under pressure from financial regulators across the world. Regulators in Europe, the United States and Asia have called for tighter compliance controls on crypto exchanges to prevent money laundering and other illicit uses of digital currencies. VIENNA ATTACK On Nov. 2, 2020, Fejzulai, a 20-year-old Austrian who also held North Macedonian nationality, was killed by police minutes after he opened fire on crowded bars in Vienna. Armed with an automatic rifle, a handgun and a machete, he had opened fire at six places near Vienna"s main synagogue. Islamic State later claimed responsibility for the attack. In a public statement in July last year, Germany"s Fed­er­al Pub­lic Pros­e­cu­tor General said Drilon G. and Blinor S. were suspected of knowing about the attacks in advance and failing to report them to the police. The statement said special forces and BKA officials had searched the two men"s addresses in the German cities of Kassel and Osnabrueck. The prosecutor"s office, calling the men "suspected accomplices in the attack," did not mention cryptocurrencies, Binance, or any evidence they funded Fejzulai. Their lawyers confirmed to Reuters both were targets of the BKA"s criminal investigation. The pair had been in close contact with Fejzulai on social media before the attacks, the prosecutors" statement said, and in July 2020 stayed at his apartment in Vienna for several days along with Islamists from Austria and Switzerland. It highlighted their "close personal relationship with the assassin and their shared radical Islamic sentiments." Blinor S. told Reuters there was no evidence for prosecutors" claims. Drilon G. said the accusations were false and he didn"t "have anything to do with the horrible terror attack." The prosecutor"s office declined to comment, saying the investigations were ongoing. The prosecutor"s office said DNA from unspecified participants at the Vienna meetings was later found on Fejzulai"s weapons and on an Islamic State ring he wore during the attack. Just before Fejzulai began his assault on the evening of Nov. 2, Blinor S. and Drilon G. deleted communications with Fejzulai on their mobile phones and social media profiles, it said.