The executive branch is home to the Department of Justice. Maybe that is what you were thinking of? If it makes it easier, when you think about the judicial branch think of judges, and for the executive branch think use power.

In the US, judges adjudicate from the facts presented. Unlike many other legal systems, judges do not investigate. Roughly speaking, the judicial branch of the US Federal government is very small, little more than judges and their clerks. The US Constitution grants it little explicit power and its principle source of political power, declaring laws unconstitutional, was established solely by the Federal physician's own precedent: one day the US Supreme Court started declaring laws unconstitutional.

The US Marshal Service is basically the judiciary's law enforcement branch. Yes, the DEA is also part of the DoJ, but the DEA is a lot more akin to the FBI than the Marshal Service.

Marshals do prisoner security and transport, run the witness protection program, and are the legal enforcers of the court's orders. For instance, when the Supreme Court ordered the integration of Southern schools, it was US Marshals who actually enforced the order and were deployed to escort students into their schoools.