The charges announced Monday in the special counsel investigation into Russian meddling in the U.S. election signal an aggressive probe -- a double-barreled punch that veteran prosecutors say is just the beginning, CBS News' chief legal correspondent Jan Crawford reports.

Taken together, lawyers say the indictment of Paul Manafort and Rick Gates, and the plea deal with George Papadopoulos, have a common purpose beyond the underlying crimes: to shake some trees and get witnesses to cooperate with the broader investigation.

Papadopoulos met with government lawyers and started cooperating after his arrest in July. Manafort and Gates, so far, have not.

But now, the threat of prosecution and jail time -- or even additional charges against them -- could cause them to flip if they have information that special counsel Robert Mueller would be interested in.

"Extremely strong, very serious charges, that put a tremendous amount of pressure on Manafort and Gates to plead and cooperate," former federal prosecutor Peter Zeidenberg told CBS News correspondent Julianna Goldman.

That's a common approach in these wide ranging investigations, Crawford reports. They're not just one big case that's filed at the very end, but a series of interim cases throughout.

"This investigation has got a head of steam and there will be a lot more shoes to drop," Zeidenberg said.