This is a long article about the FBI’s Data Intercept Technology Unit (DITU), which is basically its own internal NSA.

It carries out its own signals intelligence operations and is trying to collect huge amounts of email and Internet data from U.S. companies — an operation that the NSA once conducted, was reprimanded for, and says it abandoned.

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The unit works closely with the “big three” U.S. telecommunications companies — AT&T, Verizon, and Sprint — to ensure its ability to intercept the telephone and Internet communications of its domestic targets, as well as the NSA’s ability to intercept electronic communications transiting through the United States on fiber-optic cables.

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After Prism was disclosed in the Washington Post and the Guardian, some technology company executives claimed they knew nothing about a collection program run by the NSA. And that may have been true. The companies would likely have interacted only with officials from the DITU and others in the FBI and the Justice Department, said sources who have worked with the unit to implement surveillance orders.

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Recently, the DITU has helped construct data-filtering software that the FBI wants telecom carriers and Internet service providers to install on their networks so that the government can collect large volumes of data about emails and Internet traffic.

The software, known as a port reader, makes copies of emails as they flow through a network. Then, in practically an instant, the port reader dissects them, removing only the metadata that has been approved by a court.

The FBI has built metadata collection systems before. In the late 1990s, it deployed the Carnivore system, which the DITU helped manage, to pull header information out of emails. But the FBI today is after much more than just traditional metadata — who sent a message and who received it. The FBI wants as many as 13 individual fields of information, according to the industry representative. The data include the route a message took over a network, Internet protocol addresses, and port numbers, which are used to handle different kinds of incoming and outgoing communications. Those last two pieces of information can reveal where a computer is physically located — perhaps along with its user — as well as what types of applications and operating system it’s running. That information could be useful for government hackers who want to install spyware on a suspect’s computer — a secret task that the DITU also helps carry out.

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Some federal prosecutors have gone to court to compel port reader adoption, the industry representative said. If a company failed to comply with a court order, it could be held in contempt.

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It’s not clear how many companies have installed the port reader, but at least two firms are pushing back, arguing that because it captures an entire email, including content, the government needs a warrant to get the information. The government counters that the emails are only copied for a fraction of a second and that no content is passed along to the government, only metadata. The port reader is designed also to collect information about the size of communications packets and traffic flows, which can help analysts better understand how communications are moving on a network. It’s unclear whether this data is considered metadata or content; it appears to fall within a legal gray zone, experts said.

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The Operational Technology Division also specializes in so-called black-bag jobs to install surveillance equipment, as well as computer hacking, referred to on the website as “covert entry/search capability,” which is carried out under law enforcement and intelligence warrants.

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But having the DITU act as a conduit provides a useful public relations benefit: Technology companies can claim — correctly — that they do not provide any information about their customers directly to the NSA, because they give it to the DITU, which in turn passes it to the NSA.