LONDON — “We’re not banning you, we’re just not allowing you access,” security chief Mike Oldknow told me, when I showed up Wednesday to attend one of the world’s largest annual counterterrorism events.

The Counter Terror Expo is held in a large conference hall in Kensington (pictured above) on the west side of London. Hundreds of companies and government officials meet there every year to discuss the latest developments in the broad field of national security.

It was an unexpected situation at a gathering I had covered several times in the past. As in previous years, I registered in advance for press accreditation and, as normal, had my application approved. “Please bring your badge reference number and photo ID,” I was told in an email. “Hope you have a great show.”

Supported by the U.K. government, the event regularly attracts big multinational defense contractors like Lockheed Martin, Northrop Grumman and General Dynamics. It is also attended by many smaller, lower-profile companies offering up a variety of controversial covert surveillance tools, biometric technologies, drones and other security equipment.

This year, a host of talks were scheduled, including one about domestic extremism from a top London police officer, and another from a NATO official about “the changing nature of international terror.”

But problems began as soon as I arrived. At the media registration desk, I handed over my badge reference information, and a woman named Georgia entered the details onto a computer. She frowned, looked at me, and said she would have to make a couple of phone calls.

A few minutes later, her colleague arrived on the scene. There was a quiet discussion between them behind the desk, and then he too made a few calls. I heard him saying on the phone: “The Intercept? What is The Intercept?”

It was interesting and surprising to hear the employees talking about my association with The Intercept while considering whether to let me in — I had registered to attend as a freelancer and had not included mention of the site in my application.

Eventually, Georgia turned to me and explained that my pass had been rejected because of a decision made by someone “higher up.” She wouldn’t tell me any other details. I asked for an explanation, since I’d attended in previous years without any problems.

She said she was going to have to call the head of security.

A few minutes later, Oldknow, the security chief, arrived. He wore a dark suit and swaggered up toward the media desk, flanked by several tall, angry-looking security guards.

Oldknow ushered me over to the side of the room. I pressed the red button on the voice recorder I was holding and asked why my pass had been suddenly revoked.

“I don’t have a reason, I’m just in charge of security,” he said. “I enforce the rules; I don’t make them.”

He explained that the event’s media partner had approved the registration.

“But someone else has overruled that,” he said. “And on the system it says you are rejected as press.”

So who is kicking me out and why?

“I can only assume it’s because you’re not what they would deem … well, I wouldn’t use the word professional … but there are journalists that are focused on defense and security and they are the people that they invite to this event.”

I told him I was focused on defense and security, that I had received an invite, and that I’d attended in previous years without any issues. He had no answer.

“I hope you’re not recording this?” he asked, glancing down at my voice recorder.

I told him I was.

He looked a bit startled, refused to utter another word, and pointed me to the door.

I left, and later phoned the event’s media representatives to seek further explanation. None of them could tell me why my pass had been revoked. After I tweeted about the incident, one of the representatives called me and said he was sorry for what happened, but he still had no explanation for why my pass was revoked, or at whose request.

Back inside the event, the organizers were serving up breakfast for accredited reporters, and hosting a “networking drinks reception” in the VIP area for members of the press and counterterror company executives to mingle. Perhaps one of the organizers wanted to prevent any unruly investigative journalists from gate-crashing the schmooze fest and ruining the family atmosphere.

The incident felt symptomatic of the excessive secrecy plaguing the counterterrorism industry, which is worth tens of billions of dollars every year but operates almost entirely in the shadows. The only thing that seems to truly terrorize the industry is the prospect of transparency and public accountability.

Welcomed with open arms are the journalists who write puff pieces about the latest counterterror technology for defense industry magazines. But veer too far from that script, expose the industry to some proper scrutiny, and you can expect to be kicked out the door.

Photo of the Counter Terror Expo in London in 2011. (Ryan Gallagher)