'This Week' Transcript: Gen. Martin Dempsey, Reps. Ruppersberger and King, and Glenn Greenwald

New York, August 4, 2013— -- RADDATZ: Good morning. Welcome to This Week.

Cause for alarm: there is a worldwide alert. We have breaking details this morning on that terror threat. New concerns about that risk to the homeland, plus the fresh intelligence that has officials so spooked and so convinced al Qaeda may be poised to strike.

Then, our exclusive interview with the president's top military adviser. Will Edward Snowden's secrets end up in enemy hands?

And the man at the center of the domestic spying debate, Glenn Greenwald, reveals his brand new report here live.

Plus.

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ANNOUNCER: From ABC News, "This Week with George Stephanopoulos" starts now.

RADDATZ: Good morning. I'm Martha Raddatz. George has the morning off. It's great to have you with us. And we begin with breaking news as more than 20 embassies and consulates are closing around the world right now. And here at home, increased security measures are now in place. ABC News has learned this morning that the intercepted communications that led to the alert indicate terrorists are planning an attack that is going to be big and, quote, "strategically significant."

Yesterday the White House held an hour's long meeting, high-level meeting with the country's top national security official to discuss the response to the threat. And we've just learned what went on at that meeting.

So let's go straight to Jon Karl who is at the White House. And Jon, it sounds like the national security community is really spooked by this.

JON KARL, ABC NEWS CORRESPONDENT: No doubt about that, Martha. The high-level meetings here at the White House over the weekend are a sign of just how seriously the U.S. is taking this threat.

In fact, officials tell us they believe that they are al Qaeda operatives already in place for this attack in Yemen and possibly in other countries, as well.

The cause for concern are those intercepted communications from the leadership of the al Qaeda affiliate in Yemen.

One U.S. official telling us, quote, "the part that is alarming is the confidence they showed while communicating and the air of certainty about their plans."

The official tells us they even talked about their media plan for after the attacks take place.

Now, one of the things that is especially concerning about the al Qaeda affiliate in Yemen is that they have developed techniques to evade western security measures, specifically officials are concerned about terrorists carrying surgically implanted bombs.

As one U.S. official told us, quote, "these are guys who have developed the techniques to defeat our detection methods."

RADDATZ: Jon, people I've talked to said they don't really know exactly what the target even if it's an embassy or consulate. What do you know about that?