The Wall Street Journal, which broke the story Thursday, suggested Mueller had impaneled the grand jury weeks ago as part of the probe into possible ties between Trump campaign associates and the Kremlin during last year’s election.

Charlie Gasparino, a senior correspondent for Fox Business Network and a former WSJ reporter, called the impanelment a “significant step” in the investigation during an appearance Thursday on Fox News.

“Let me tell you something,” Gasparino told Fox News’ Neil Cavuto. ”[Mueller] wouldn’t have impaneled a grand jury unless he thought there was something there.”

“I mean this is serious,” he added. “I don’t know how else to put it.”

The WSJ report follows Mueller’s appointment this week of several high-powered lawyers. Reuters reported Tuesday that Greg Andres, a former Justice Department attorney who specialized in corruption and bribery cases, joined the investigation.

Gasparino, noting his experience covering white-collar crimes and grand jury proceedings as a reporter, said prosecutors “generally” don’t impanel a grand jury over “random stuff.”

“It’s actually an overt step that there is an investigation, meaning that ... there’s smoke there and we have to see if that smoke is fire,” Gasparino said.

CORRECTION: An earlier version of this article misstated that Charlie Gasparino’s title. He is a senior correspondent for Fox Business Network, not a Fox News contributor.