The grand jury was impanelled in July 2017 for a standard 18-month term and was set to expire this week before Howell's extension.
The grand jury's term can not be extended after the six months are up. Mueller and his team are investigating possible Russian Federation collusion and obstruction of justice by President Trump.
The extension was approved by the chief justice of the D.C. District Court, who oversees Mueller's grand jury.
Mr. Mueller is using the 23-person federal grand jury to conduct his investigation into Russian interference and other matters, leading to the indictment of some of President Trump's campaign officials. Their work has resulted in multiple criminal cases against people connected to Mr. Trump, including former campaign chairman Paul Manafort, and campaign adviser George Papadopoulos.
The grand jury's work is secret.
"It would be burdensome to put that testimony before a new grand jury", because sometimes that testimony must be imparted in the form of transcripts read to the new panel, absent context and a live witness's demeanor, former Chicago federal prosecutor Renato Mariotti said.
All three have pleaded guilty.