The National Security Agency (NSA) has been involved in various surveillance programs over the years, some of which have involved monitoring of US citizens. Here is an outline of some of the more well-known programs:

1. PRISM: This program, which began in 2007, involved the collection of data from major technology companies such as Google, Microsoft, Facebook, and Apple. The data included emails, photos, and documents, as well as audio and video chats.
2. Upstream Collection: This program involves the interception of internet traffic as it flows through the cables and switches that make up the internet’s infrastructure. This allows the NSA to collect both the content of communications as well as metadata (such as the “to” and “from” addresses).
3. Section 702: This provision of the Foreign Intelligence Surveillance Act (FISA) allows the NSA to collect communications of non-US persons outside of the US, but also incidentally collects communications of US citizens or residents.
4. XKeyscore: This program allows analysts to search and analyze vast amounts of internet traffic, including emails and chats, without a warrant. The use of National Security Agency (NSA) gathered intelligence in law enforcement investigations raises important legal and constitutional questions, particularly with regards to the Fourth Amendment’s prohibition on unreasonable searches and seizures.

The Fourth Amendment requires that any search or seizure be supported by a warrant issued upon probable cause, which must be supported by oath or affirmation, and describe with particularity the place to be searched and the persons or things to be seized. The amendment is designed to protect individual privacy and prevent arbitrary government intrusion.

When it comes to the use of NSA gathered intelligence in criminal investigations, there are several legal and constitutional issues to consider. One important question is whether the government’s collection of the intelligence violated the Fourth Amendment’s protections against unreasonable searches and seizures.

In general, the government must obtain a warrant supported by probable cause to conduct a search or seizure of an individual or their property. However, there are certain exceptions to the warrant requirement, such as the exigent circumstances exception or the consent exception.
When it comes to NSA gathered intelligence, the government argues that it is not gathered with the purpose of law enforcement but rather national security and also is not gathered specifically as a means to circumvent a person’s Fourth Amendment rights.

However, if the NSA gathered intelligence is used in a criminal prosecution, the government must show that it was obtained in compliance with the Fourth Amendment. This means that the government must establish that any search or seizure was reasonable under the circumstances and supported by a valid warrant or exception to the warrant requirement.

In addition to the Fourth Amendment issues, the use of NSA gathered intelligence in criminal investigations raises questions about due process and the right to a fair trial. If the government relies on classified intelligence in a criminal prosecution, it may be difficult for the defendant to challenge the evidence or cross-examine the government’s witnesses.

Overall, the use of NSA gathered intelligence in criminal investigations is a complex and controversial issue. While the government argues that it is necessary for national security and public safety, it also raises important legal and constitutional concerns. As such, the use of such intelligence must be carefully scrutinized to ensure that it is consistent with the protections of the Fourth Amendment and other constitutional rights.