The Twitterverse exploded with questions. Couldn’t the NSA break open the phone? If it could, why didn’t it?

But notably missing from the FBI’s argument was any mention of whether it had consulted spies and sleuths from the government’s intelligence community — particularly the National Security Agency.

The bureau told a judge in February that Apple has the “exclusive technical means” to try to unlock the contents of San Bernardino shooter Syed Rizwan Farook’s iPhone — and that’s why it should be forced to do so.

I hear the @FBI made a sworn declaration that there are no alternative means to unlock the #Apple phone. Anyone have a cite? It's important.

Reporters, if you talk to DOJ or the FBI about Apple, please ask them why the FBI hasn't sought the NSA's help. https://t.co/RBnn99BChK

Apple itself raised those questions in a court filing. “The government has not made any showing that it sought or received technical assistance from other federal agencies with expertise in digital forensics, which assistance might obviate the need to conscript Apple to create the back door it now seeks,” the company’s attorneys wrote.

The NSA, after all, has long targeted digital encryption systems for exploitation, and, as The Intercept revealed in 2015, the CIA and NSA have been working for nearly a decade specifically to find ways to hack into Apple devices. Those agencies could presumably help the FBI do what it wants to do to Farook’s iPhone: place a modified version of Apple’s iOS operating system on the device that allows rapid, unlimited attempts to guess Farook’s encryption passcode.

Peter Thomson, a former federal prosecutor who worked on special assignment at the NSA, told The Intercept, “I know of no case law that would put the burden on the FBI to go to the intelligence community,” adding, “I don’t think the NSA has to share what it can and can’t do.”

“Apple is being quite creative with its argument. Assuming first that the NSA can get into the phone — then [asking it to help] is pushing the envelope into the classified world,” Thomson said.

FBI Director James Comey, when asked directly during a House Judiciary Committee hearing on Tuesday if the bureau had tried to get help from “agencies such as the NSA,” replied, “Yes is the answer. We’ve talked to anybody who will talk to us about it.”

He later said: “We don’t have the capabilities that people sometimes on TV imagine us to have. If we could have done this quietly and privately, we would have done it.”

But his lack of specificity — What did the NSA say when contacted by the FBI? Who is “we”? — did not really clear things up.

So why didn’t the NSA help? Here are a few possibilities:

1. The NSA tried to help, but it couldn’t.

This seems unlikely, but it’s possible. For instance, it’s conceivable the agency isn’t able to forge Apple’s cryptographic signature, which is required to install a modified operating system on an iPhone.

Dan Guido, CEO of the security research firm Trail of Bits, explained in a blog post: “Before any firmware image is loaded by an iPhone, the device first checks whether the firmware has a valid signature from Apple. This signature check is why the FBI cannot load new software onto an iPhone on their own — the FBI does not have the secret keys that Apple uses to sign firmware.”

2. The NSA could help, but doesn’t want to.

“They don’t want to acknowledge they have the capability at all,” said Chris Soghoian, principal technologist at the American Civil Liberties Union. “It’s giving the NSA’s adversaries false confidence. If you’re Angela Merkel and you have an iPhone, you’re feeling pretty good right now.”

Similarly, if Apple knew the NSA had found a vulnerability, it would presumably try to fix it.

Austin Berglas, a former FBI agent now at K2 Intelligence, told BuzzFeed the NSA might fear its help could end up being exposed in court. “There are capabilities that the U.S. government has, that are used for intelligence collecting only and that wouldn’t be used for a criminal matter because they would have to come up in open court.”

But Farook’s phone is unlikely to yield any useful evidence — and particularly unlikely to spark new legal proceedings.

3. The NSA isn’t allowed to help.

The NSA’s mandate is to gather foreign intelligence while protecting U.S. secrets in the interests of national security.

But Liza Goitein, co-director of the Liberty and National Security Program at the Brennan Center for Justice, said that’s never stopped the NSA from investigating at home before. “The NSA … has a very broad interpretation of what’s foreign intelligence — broad enough to collect American phone records,” she said.

And Thomson, the former prosecutor, told The Intercept, “I’m not aware of any law that would prohibit the NSA from providing technical assistance.”

In fact, the NSA has already helped with the San Bernardino case. Adm. Mike Rogers, the NSA’s chief, told Yahoo News that his agency successfully gathered metadata records for Farook’s phone — but not content.

Indeed, the separation between outward-facing and inward-facing intelligence agencies has blurred considerably over the past 15 years. Less than three years after 9/11, then-FBI Director Robert S. Mueller said the bureau was in a much better position to prevent terrorist attacks “because we coordinate much more closely and regularly with the CIA and NSA … because the legal wall between intelligence and law enforcement information has been eliminated.”

4. The FBI doesn’t want the NSA’s help.

The most cynical explanation for why the NSA hasn’t helped is that the FBI hasn’t really tried to get it to. “If they ask the NSA for help,” said Soghoian of the ACLU, “they don’t get the precedent they want.”

Though Comey told Congress that the FBI has “talked to anybody who will talk to us about it,” there’s little question that the FBI would rather force technology companies to develop these kinds of tools so that law enforcers across the country can use them, rather than have to ask the NSA for help on a case-by-case basis.

And while the FBI insists that the San Bernardino case isn’t about “sending a message” or establishing a legal precedent, law enforcers across the country have said this case could fundamentally change the way they investigate crime. Apple is currently fighting 12 other court cases where it’s resisting orders to unlock a phone — some of them very early generations of iPhones with operating systems that are much less secure.

The House Judiciary Committee’s ranking Democrat, Rep. John Conyers, expressed concerns earlier this week that the government is exploiting the tragedy in San Bernardino to push its agenda on encryption while sidestepping congressional debate.

“I would be deeply disappointed if it turns out that the government is found to be exploiting a national tragedy to pursue a change in the law,” he said during a hearing on Tuesday.