Ordinarily, police departments cannot access personal bank account information, which is protected by key privacy rights in the United States (laws for accessing banking information may work differently in the UK, for instance). However, there is a branch of criminal investigation known as forensic accounting, which deals in examining banking and other financial information to spot malfeasance. The law allows this type of inquiry in specific circumstances.
As technology and banking becoming more intertwined, hacking attempts are becoming increasingly common. In order for law enforcement to track down the hackers, they have to trace the hack back to a source. This means they have to start at the account or bank that was hacked, which may give them access to private software and personal account information in the process.
Identify theft occurs when someone steals identify and financial information to steal money. Identity theft may involve the theft of credit or debit cards and usually allows the thief to access bank accounts. To trace the theft and attempt to catch the thief, law enforcement organizations start by examining banking information, including what they bought and how the money was withdrawn from the account.
There are many types of fraud, and most types require the investigation of bank accounts. Business fraud occurs when companies lie about themselves or their financial standing. The only way to ascertain the extent of the fraud is through forensic accounting, examining the business accounts and activities associated with them. This is how leaders at Enron were convicted.
Law enforcement organizations also look at account information when tracking illegal drug activities, especially the large-scale creation and selling of drugs. While the money gathered from these activities is rarely reported as taxable income, it is often stored in bank accounts, giving law organizations a means to track profit and sales.