Robert Mueller has something new cooking.

In a new court filing on Thursday, the special counsel’s office revealed additional details of the probe that indicate he has recently expanded his investigation of Paul Manafort. The further implication of this filing is that Mueller is actively building a collusion case against the former Trump campaign chairman or other Trump campaign officials, and potentially basing it on the testimony of former Manafort deputy Rick Gates.

The new details show that Mueller’s team acquired search warrants on five telephone numbers last month, just two weeks after Gates began to officially cooperate in Mueller’s probe.

The filing was a response to a motion from Manafort’s attorneys to see additional details of search warrants related to Manafort. For the most part, Mueller’s team has turned over these details. But as it pertained to a warrant obtained on March 9 for the phone numbers, the special counsel’s office insisted that the warrant be redacted because they are “relating to ongoing investigations that are not the subject of either of the current prosecutions involving Manafort.”

As Mueller laid out in another response to Manafort’s attorneys earlier this week, Deputy Attorney General Rod Rosenstein last August gave him authority to investigate Manafort on at least two fronts. First was whether Manafort “[c]ommitted a crime or crimes by colluding with Russian government officials with respect to the Russian government’s efforts to interfere with the 2016 election for President of the United States.” Second was, whether he “[c]ommitted a crime or crimes arising out of payments he received from the Ukrainian government before and during the tenure of President Viktor Yanukovych.”

If the warrants against the five phones are “not the subject of either of the current prosecutions involving Manafort,” then they would seem to not be connected to the crimes arising out of the Ukrainian imbroglio, which is the subject of his current prosecutions. By process of elimination, they are related to either a potential collusion case against Manafort, or additional targets. If there are additional targets, though, they would have to be connected in some way to Mueller’s wider mandate to investigate Russia’s interference in the 2016 election and/or the Trump campaign’s potential cooperation with that interference and links to Russia.

In other words, Mueller appears to be going after someone for collusion using the Manafort connection.

Legal analyst Renato Mariotti observed on Twitter that this new warrant could very well mean that Mueller is seeking “historical cell site information” for Manafort’s old phones, which would mean he’s trying to track the timing of Manafort’s previous movements. Again, this all came soon after Gates began cooperating with the investigation. Gates has already pleaded guilty and confessed to lying in February to the FBI about his knowledge of a meeting between Manafort, Mercury Public Affairs lobbyist and former Congressman Vin Weber, and Rep. Dana Rohrabacher at the Capitol Hill Club in 2013. (Republican House Majority Leader Kevin McCarthy was recorded in 2016 saying that he—“swear to God”—believed Rohrabacher was on Putin’s payroll, which his staff first denied him saying and then argued was “clearly an attempt at humor.”)

Whether or not Rohrabacher is involved in the investigation, it’s clear that Gates was privy to the scheduling and substance of some of Manafort’s meetings. He was also lying about them to investigators during proffer sessions up until two months ago. If Gates is being more forthcoming about these meetings now, that might explain why Mueller would have the probable cause he might feel he needed to track Manafort’s old movements.

Whatever the answer is here, it seems clear that since Gates began cooperating, Mueller has gleaned what he deems useful information—and what the court system deems “probable cause” to expand his inquiry.