The special counsel Robert Mueller was appointed one year ago today to investigate Russian meddling in the US election and possible ties to President Donald Trump's campaign.

Trump and his allies have recently stepped up the pressure on Mueller to wrap up the investigation.

Special prosecutor investigations take an average of 904 days, much longer than Mueller's investigation.

One year ago today, former FBI director Robert Mueller was appointed as the special counsel charged with investigating possible ties between President Donald Trump's campaign and Russia.

In the year since, Mueller's team has handed out a series of indictments and interviewed dozens of witnesses. Meanwhile, Trump has railed against Mueller, calling into question everything from the special prosecutor's team to the legality of the whole investigation.

Recently, Trump allies including Vice President Mike Pence and Trump lawyer Rudy Giuliani, both publicly requested that Mueller finish the investigation.

"Our administration has been fully cooperating with the special counsel," Pence said in an interview with NBC. "What I think is that it's been about a year since this investigation began. ... In the interest of the country, I think it's time to wrap it up."

But the length of Mueller's investigation isn't atypical of special prosecutors in the past.

A chart from Compass Point's Issac Boltansky and Lukas Davaz shows that the average length of special counsel investigations, dating back to Watergate, is 904 days.

The longest a special counsel has maintained an investigation was the look into President Bill Clinton's Whitewater real-estate deals. That investigation eventually morphed with the Monica Lewinsky scandal. It went on for 2,978 days.

The investigation into the arms deals around the Iran-Contra affair under President Ronald Reagan went on for 2,420 days.

It has been 365 days since Mueller was appointed.