(CNN) The special counsel's office has accused former Trump campaign chairman Paul Manafort of attempting to get witnesses to lie for him in court, and they've asked the judge to send him to jail as he awaits his trial, according to a filing in DC District Court Monday night.

One of the witnesses told investigators recently that Manafort wanted them to commit perjury about a lobbying effort they worked on for him in the US, the filing said.

Manafort is currently out on house arrest and a $10 million unsecured bail. He awaits a trial in Virginia that is scheduled for late July and a trial in DC scheduled to begin in September. He has pleaded not guilty to charges related to his failure to disclose his US lobbying work for a foreign government and to bank fraud and other financial crimes.

Tampering with a witness is a crime in itself -- and it's one Manafort has not yet been charged with.

The new allegation places even more pressure on Manafort, who has stared down prosecutors as they heaped charges on him and say they continue to investigate the possibility that he coordinated with Russians during the 2016 presidential election.

Jason Maloni, a spokesman for Manafort, said Tuesday that the former campaign chairman was innocent.

"Nothing about this latest allegation changes our defense. We will do our talking in court," Maloni said.

Manafort must respond more thoroughly by Friday in court to the special counsel's office allegations that he attempted to tamper with witnesses before his trial. He and Mueller's prosecutors will also discuss the allegation and whether he should be imprisoned for violating his bail terms at a hearing June 15. At that hearing, an FBI agent who tracked Manafort's communications with the potential witnesses and the witnesses themselves, who have not been named, may testify, the federal judge in the case, Amy Berman Jackson, said Tuesday morning.

Hapsburg group activity

In late February, the new filing says, Manafort "repeatedly contacted" two anonymous people who may be witnesses against him, called Person D1 and D2. Those two people had previously assisted lobbying and public relations efforts that Manafort masterminded in the US and Europe. They have recently helped investigators find evidence of the witness tampering effort, prosecutors said.

Manafort and a confidante first asked the two people to claim that the Ukrainian lobbying work only happened in Europe, effectively attempting to get their stories all on the same page, since all involved knew the group had lobbied Congress, investigators said. Manafort also attempted to use the two people to influence the distinguished group of former European political leaders they previously worked with, called the Hapsburg group, who were part of the alleged lobbying scheme.

The two people had previously helped Manafort arrange for the Hapsburg group to contact US senators, meet with members of Congress and their staffs, and publish an op-ed in The Hill newspaper in Washington that supported the politics of former Ukrainian President Viktor Yanukovych, while posing as independent voices, prosecutors have said. In Monday night's filing, prosecutors attached several emails and schedules from 2011 to 2013 about the Hapsburg group's meetings in the US with members of Congress.

Manafort's allegedly illegal contacts with the two people happened in the days after prosecutors unveiled a new set of criminal charges against him in DC related to his lobbying work. Those charges had revealed for the first time the existence of the Hapsburg group and its efforts in the US.

Also that week, on February 23, Manafort's longtime deputy Rick Gates agreed to plead guilty to charges of conspiracy and lying to investigators and would cooperate with prosecutors, making Manafort's case an even more difficult one to overcome.

WhatsApp messages and phone calls

Manafort's contacts with the two potential witnesses started benignly enough, with phone calls and text messages through the service WhatsApp.

"This is paul," the first encrypted text message that the government obtained said.

Then, two days later, he sent a news article about the Hapsburg group revelation in his court case. "We should talk. I have made clear that they worked in Europe," Manafort wrote.

He called Person D1 five times in late February.

Person D1 tried to avoid him, even hanging up on one of the calls, the prosecutors said.

Another Manafort confidante whom prosecutors have said has ties to Russian intelligence then contacted the second Manafort lobbying assistant on February 28 "out of the blue," the person told investigators, sending a "hi! How are you? Hope you are doing fine" and a wink emoji. The Manafort confidante, referred to as Person A, then told the second potential witness he wanted to reach Person D1 "to brief him on what's going on."

The confidante continued to try to spread Manafort's message, by sending an encrypted text that outlined Manafort's "quick summary" that "our friends never lobbied in the US."

Because witness tampering is a crime, and Manafort's bail provisions warn that he shouldn't break the law while awaiting trial, the prosecutors argue they have "little confidence that restrictions short of detention will assure Manafort's compliance," the filing said.

Manafort currently wears two GPS monitor ankle bracelets on separate legs, keeping him confined to his Alexandria condo except for legal, medical and religious appointments. He has attempted to post $10 million in properties as his bail several times since his initial arrest in late October, so he could move about more freely, to no avail.

Prosecutors also submitted the filing they made in DC to the judge overseeing Manafort's financial crimes case in Virginia, T.S. Ellis, in case he wants to change Manafort's bail terms there.