Since Secret Service Agents perform both criminal investigative and protective work, we'll touch on both functions here. Agents wield a lot of power when investigating current and potential criminal cases. First, Secret Service agents can legally pack a firearm. Agents can also execute warrants legally issued under United States law, and can arrest people without warrants if the alleged perpetrator commits an offense against the United States in front of the agent. An agent can even arrest someone for a felony if the agent reasonably believes the subject has committed that crime.
A Secret Service agent can post and pay a reward for information, plus other unspecified services, that lead to the arrest of the miscreant who allegedly violated the law. Regardless of what the agent has on his plate, he partners with the United States Attorney's Office to make sure he performs his duties in accordance with the law.
Now we get to an agent's protective functions, which are probably not much of a secret to anyone. However, you might wonder what power Secret Service agents can flex to keep the President, plus everyone else on the list, out of harm's way. Can the agent close streets around a Presidential appearance? Yep. Can an agent set up a security perimeter, which might include snipers on rooftops and welded-shut manhole covers and mailboxes? Yep, he can do that, too. Agents can insist on maddeningly inconvenient security checkpoints, alterations in speaking venues, and pretty much anything else they feel is necessary. Money is no object when it comes to keeping a protectee safe...and your tax dollars pay for it.