As security preparations intensify for Charlie Kirk’s memorial service in Arizona, a former NYPD detective and security consultant warns mourners to remain alert."Pull your face out of your phone, put it in your pocket. Pay attention," Pat Brosnan urged. The memorial, set for this weekend at State Farm Stadium, is expected to draw an enormous crowd. Officials estimate that more than 100,000 people may gather in and around the venue, including attendees inside the 63,000-seat stadium and overflow areas outside."From the moment you arrive… have your eyes wide open… who's to your right, who's to your left, who's behind you? What are they doing?" Brosnan told Fox News Digital. "Eyes in the backs and the sides of your head."TRUMP’S SECURITY UNDER SCRUTINY AS SECRET SERVICE BRACES FOR KIRK MEMORIALAccording to a law enforcement bulletin obtained by ABC News, authorities are "tracking several threats of unknown credibility.""Violent extremists and unaffiliated lone offenders may view the memorial service or related events as attractive attack targets due to the attendance of these individuals, other senior U.S. government officials, state and local government officials, and political activists and due to major international media attention," the memo reportedly says.The Department of Homeland Security has designated Kirk’s memorial service a Level 1 Special Event, the highest possible rating, matching Super Bowl-level security planning. Officials state this designation is "one step below National Special Security Event (NSSE) which would be Olympic security or inauguration." CHARLIE KIRK FUNERAL RECEIVES HIGHEST SECURITY DESIGNATION FROM FEDERAL OFFICIALS AMID REPORTS OF THREATSAmong those expected to pay respects are President Donald Trump, Vice President JD Vance, Secretary of State Marco Rubio, and other prominent political figures.The U.S. Secret Service is now the lead agency, coordinating efforts across state and local law enforcement. Multiple drone units, aerial surveillance, and magnetometer checkpoints will be deployed throughout the State Farm Stadium area."Even while at the stadium… there could be someone in that stadium who was bent on evil," Brosnan warns. "The collective eyes and ears of 63,000 – that’s 126,000 eyes, 126,000 ears – they could see or hear something that could be critical."Brosnan, who now advises on national-level security planning, says the most important threat indicator isn't what someone looks like, but what they're doing."Not just appearance… but behavior," he says. "If you see suspicious behavior, anything that you think is anomalous or weird or out of character… trust your instincts. Notify security."His core advice: "See something, say something."TRUMP, VANCE, OTHER PROMINENT POLITICAL FIGURES TO SPEAK AT SUNDAY STADIUM EVENT HONORING CHARLIE KIRK"It’s a million times better to report and be wrong than to just suppress it," Brosnan added. "Don’t be ashamed if you’re wrong. No one will chastise you."Brosnan emphasized that the most vulnerable zones at any large-scale event are not always inside the venue — but just outside of it."The parking lot is an absolute petri dish for bad behavior," he cautioned. "They could perhaps do it in the parking lot before they get into the very high-level [security checkpoint]."He warned that attackers often exploit areas just outside perimeter security, such as parking garages, sidewalks, or even transit hubs."Any of those evil acts could be perpetrated even near the stadium, within miles, within blocks," he said. "That’s why you just have to be on your game at all times."RED HATS AND CHANTS OF 'USA' MARK VIGIL HONORING SLAIN CONSERVATIVE LEADER CHARLIE KIRK: 'ACT'Despite the full weight of federal and local security coordination, Brosnan was realistic about the limitations of modern threat detection."There are weapons that could get past even the most technologically efficient systems out there," he said. "The reality is, [those weapons] could get in there."Which is why Brosnan says public vigilance is not optional — it’s essential.More than just a technical or tactical issue, Brosnan says situational awareness is about shared responsibility."There could be someone near you who hears something, sees something — and they have to report it immediately, immediately, immediately," he stressed. Kirk is survived by his wife, Erika Kirk, and their two young children. Erika, along with the president, vice president, and other dignitaries, is expected to speak at the upcoming service.Kirk was fatally shot last week while addressing an event hosted by the Turning Point chapter at Utah Valley University in Orem, Utah.The accused, 22-year-old Tyler Robinson, is currently in custody without bail and faces multiple charges, including murder.