The move means the probe is intensifying and could stretch "for months," according to the newspaper. Impaneling a grand jury suggests Mueller "believes he will need to subpoena records and take testimony from witnesses," the Journal said.
It does not necessarily mean he will bring charges against Trump allies, however it proves Mueller believes that there is sufficient evidence that a crime was committed to warrant a criminal investigation.
Only a grand jury can issue an indictment, which is the only way that someone can be charged with committing a felony pursuant to the U.S. Constitution. A grand jury, which consists of 16 to 23 people, is an important tool that allows prosecutors to issue subpoenas that require people to produce documents and other evidence. The work that grand juries do is secret, which means that grand jurors—who are ordinary citizens chosen at random and vetted by the federal district court—cannot share what is happening before the grand jury. Federal rules also prevent prosecutors from disclosing what happens before the grand jury. Subpoenas can also be used to compel people to testify under oath before the grand jury. Mueller and his team may be expected to issue many subpoenas in the months ahead.
Former FBI Director Mueller is investigating Russian efforts to influence the 2016 election and whether the Trump campaign colluded with the Kremlin. The investigation has dogged and frustrated President Donald Trump during his first six months in office.