By Aruna Viswanatha WASHINGTON (Reuters) – The Justice Department will have to be creative in order to make sure criminals do not use virtual currencies such as Bitcoin to secretly move funds, Attorney General Eric Holder said on Tuesday. The scrutiny at the highest levels of the Justice Department comes at a time when investors have flocked to the currencies but recent incidents have called into question their long-term viability. The use of electronic currencies has captured the attention of regulators who have been unsure how to react to their growing use instead of government-issued money. Agencies ranging from the New York bank regulator to the Commodity Futures Trading Commission have been studying electronic currencies to determine whether and how they should be regulated.