Robert Mueller

Federal prosecutors have lots of ways to intensify pressure on the people they're investigating, from early morning FBI raids to leaning on relatives of those under government scrutiny.

But even by those measures, the special counsel investigating Russian interference in last year's presidential election is moving with unusual speed and assertiveness, according to half a dozen legal experts following the probe.

Updated at 8:30 p.m. ET

Special counsel Robert Mueller is using a grand jury in Washington, D.C., in connection with his investigation into Russian efforts to influence the 2016 presidential election and into possible collusion between Russia and top aides to the Trump campaign, a source with knowledge of the investigation confirms to NPR's Peter Overby. The source did not want to be identified because of the sensitivity of the matter.