A more thorough explanation:

The Federal Bureau of Investigation (FBI) is a part of the United States Department of Justice. Its main responsibility is to investigate any violations of federal laws, except for those that are assigned to other federal agencies.

For example, if someone commits a crime that involves crossing state lines, the FBI would be responsible for investigating it. However, if the crime is related to taxes, the Internal Revenue Service (IRS) would be responsible for investigating it.

The FBI has a wide range of responsibilities, including investigating terrorism, cybercrime, organized crime, and public corruption. They also work closely with other law enforcement agencies, both in the United States and around the world.

One example of the FBI's work is the investigation into the 9/11 terrorist attacks. The FBI played a key role in identifying and tracking down the individuals responsible for the attacks.

Another example is the FBI's investigation into the Russian interference in the 2016 United States presidential election. The FBI worked with other agencies to gather evidence and identify those responsible for the interference.