The NFL is seeking the appointment of a special investigator to address what it claims is widespread fraud of its concussion settlement with former players -- another step in a growing feud between the sides.

In a filing Friday to the U.S. District Court of Eastern Pennsylvania, the league said nearly half the claims it has received have been red-flagged for audit. Of that, more than 400 have been rejected after an independent claim administrator found them to be fraudulent.

The NFL is requesting the special investigator to assist the claim administrator and recommend sanctions.

"We want to ensure that players and their families receive the benefits they deserve," said attorney Brad Karp, whose firm represents the NFL. "Fraud threatens the integrity of the settlement and the prompt payment of legitimate claims. There is significant evidence of fraudulent claims being advanced by unscrupulous doctors, lawyers and even players. The appointment of a special investigator was specifically contemplated in the agreement and will provide important additional tools to assist the independent, court-appointed administrators in identifying fraudulent claims and related misconduct."

Lawyers for the players alleged in a filing last month that the NFL is intentionally slowing the claims process, according to the Washington Post. Dementia claims have been especially slow to be paid out. According to the Post, only six of the 1,113 claimants with a dementia diagnosis have been paid.

To date, more than 20,000 former players are part of the class eligible to share in a settlement that is uncapped but could approach $1 billion over its 65-year life. The settlement has approved 377 awards, according to its website, and is committed to more than $411 million in monetary awards.

But the process has been slower than expected because of fraud, according to the NFL. Among the examples listed in the league's claim:

• A law firm representing more than 100 players directed at least one to report for a medical test hungover and on Valium.

• A law firm representing more than 50 players used a pediatric neurologist to diagnose Alzheimer's disease. Of that group, 75 percent were diagnosed, many of whom were in their 30s and 40s.

• The same neurologist submitted identical vital signs for 21 players.

• A "disturbing pattern" of text messages and other communication in which players were coached by claims service providers to "beat the neuropsychological tests."

Christopher Seeger, the lead attorney for the players, said Friday in a statement that the appointment of a special investigator would be "appropriate." But he warned that the NFL shouldn't use the issue of fraud as an excuse to withhold payments to deserving players.

"We have previously expressed concerns about potentially fraudulent claims and agree the appointment of a special investigator is appropriate," Seeger said. "However, we will not allow this small number of claims to be used as an excuse by the NFL to deny payment to legitimately injured former players. Unlike other NFL benefits programs, this settlement is overseen by the court, and the League cannot escape its responsibility. We will make sure that former NFL players and their families receive every benefit they are entitled to under this agreement."