After speculation that consumed Washington all weekend, special counsel Robert S. Mueller III unsealed an indictment on Monday of former Trump campaign manager Paul Manafort and his longtime business partner Rick Gates, who also worked for the campaign. But the stealth bombshell of the day may end up being Mueller's other announcement, that George Papadopoulos, a former foreign policy adviser for the Trump campaign, has already pleaded guilty to lying to the FBI about attempts to broker meetings between Russian officials and the campaign. It appears Papadopoulos is cooperating with investigators.

These are the first charges brought by Mueller, who was appointed in May to investigate any links between Russian individuals or the Russian government and the Trump campaign. They represent a major step forward for his probe — and a potentially ominous sign for others who may be in Mueller's sights.

As the White House already is pointing out, the charges against Manafort and Gates don't directly relate to the Trump campaign. The indictment alleges that Manafort and Gates earned tens of millions of dollars working as agents of the Ukraine government and Ukrainian officials, concealed that work by lying and failing to file required reports about their foreign activities, and laundered millions of dollars through various corporate entities to pay for personal goods and services and evade income taxes. The bulk of the allegations concern activities that occurred before either man was working for the then-Republican candidate. The word "Trump" doesn't appear in the indictment.

But this is not particularly surprising. It's not unusual for prosecutors to build a case against a potential key cooperator involving charges not directly linked to the ultimate matter under investigation. Even unrelated charges give the prosecutor leverage to persuade the defendant to plead guilty, cooperate in the broader investigation, and provide evidence against the "bigger fish" to earn more lenient treatment in his or her own case. And pursuing Manafort makes perfect sense. With his extensive ties to Russia and his key role as chairman of the Trump campaign, Manafort potentially is a linchpin in Mueller's investigation.

There's little doubt that prosecutors ultimately hope to persuade Manafort to cooperate. The indictment suggests that so far he has been unwilling to do so. If Manafort were cooperating there typically would not be a grand jury indictment. There would simply be a statement of charges filed by the prosecutor, along with a plea and cooperation agreement.

This is what happened with Papadopoulos, who was not indicted but pleaded guilty under seal pursuant to an agreement with prosecutors. Unlike the charges against Manafort, the charges against Papadopoulos do relate directly to his contacts with Russian officials while working for the campaign. Papadopoulos may already be providing information that goes directly to the heart of Mueller's investigation.

With the Manafort indictment, Mueller is sending a powerful signal to other potential witnesses. If those facing potential charges refuse to cooperate, Mueller is not going to dither around for months or simply move on. He will not hesitate to throw the heat where appropriate. By unveiling both of these cases on the same day, prosecutors are making it clear that it's smarter to be like Papadopoulos: Cut a deal early and cooperate.

For Manafort himself, the indictment dramatically escalates the pressure to cooperate. Previously his own jeopardy may have seemed more theoretical. Now he has been booked on a federal indictment charging him with multiple felonies carrying a maximum potential penalty of 80 years in prison. Of course, he would never actually receive a sentence that severe — but numbers like that tend to focus the mind.

We don't know yet what other charges (if any) will be coming out of Mueller's office. But Monday's news in no way suggests the investigation is drawing to a close. The same grand jury may issue multiple indictments against multiple individuals, and could even return additional charges against Manafort and Gates. Future indictments and pleas, if any, likely will come on a rolling basis, as the prosecutors build cases against lower-level players, possibly persuade them to cooperate, and move up the ladder to higher-level targets.

Mueller's investigation continues to proceed aggressively, methodically and relatively rapidly. These charges likely are simply the first salvo. Their echoes will reverberate loudly through the corridors of Washington in the days ahead.