Confetti still was falling on Seattle Seahawks players celebrating their Super Bowl victory three years ago when a Homeland Security director sent the closest thing to a congratulatory email to officers.

The director’s email, sent seven minutes after the end of Super Bowl XLVIII at MetLife Stadium, said: “We encountered a few minor security incidents — all resolved and deemed nonthreatening to SB48 operations. It was a nice, safe (and thankfully warm) game.”

Super Bowl LI will be played in Houston on Sunday evening, and security again is of the utmost concern. FBI spokeswoman Christina Garza told Newsday that security planning for this Super Bowl began three years ago.

“The Super Bowl is a classic symbolic target, and that’s what the bad guys like to hit, because it puts them front and center to the world,” former FBI agent James Conway told Newsday on Tuesday.

The Super Bowl XLVIII email, along with dozens of Homeland Security and FBI documents obtained by Newsday via a Freedom of Information Act request, illustrate the seemingly endless and perhaps unenviable task of maintaining safety at the nation’s biggest sporting event.

Conway spent 15 years working in counterterrorism and now runs Houston-based consulting firm Global Intel Strategies. He said the reports obtained by Newsday show a limited window into the inner workings of security at an event such as the Super Bowl.

According to a Homeland Security summary, the incidents that were investigated during the New York-area Super Bowl included multiple false bomb threats, a threat of an aerial attack, someone threatening “to make the power go out at the Super Bowl again” (a reference to the power outage that stopped play at the previous Super Bowl) and a person arrested for flying a drone around Times Square, near where the Super Bowl fan festivities were being held.

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Most notably, the documents omit the methods by which agents investigate each lead.

“I was in the FBI a long time, and a lot of what is done is secret because we don’t want to tell the bad guys what we’re doing,” Conway said. “People will see some of the security and some of the security they will not see.”

Conway added, “We’ve been doing this for so long that it becomes more and more sophisticated and more and more detailed every year. You can rest assured that in the three years since the Super Bowl in New York, today there are more elements in place that weren’t there then.”

What those elements are, no one is saying — also by design. Garza, the FBI spokeswoman, declined to say how many officers are involved with security at the Super Bowl.

Houston Police Chief Art Acevedo said this past week that authorities are ready to handle any protests that might occur in response to President Donald Trump’s executive order on refugees and immigration.

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“I have no concerns related to that,” NFL security chief Cathy Lanier said during the NFL’s annual pregame security briefing Wednesday in Houston.

Homeland Security chief financial officer Chip Fulghum — who is the department’s second-in-command — noted that there are 41 federal agencies working with the NFL’s security team and the Houston Police Department.

The NFL said it has hired more than 4,000 private security and crowd management personnel to work at the Super Bowl.

“We have visible security and behind-the-scenes security as well,” Fulghum said. FBI special agent Perrye Turner added, “We hope you don’t see us this week, but know that we are there.”

Officials said there are no credible threats, but as the Homeland Security documents obtained by Newsday show, what often goes unnoticed is the work that takes place behind the scenes to determine that a threat is not credible. The documents list more than 100 incidents of suspicious activity that raised to the level of requiring further investigating.

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“Intelligence is really important,” said Fulghum, the Homeland Security CFO. “Our federal partners in the state and county and local intelligence agencies are doing a really good job in monitoring the threat stretch around the region, the state, the country and the world.”