The NFL said its investigators had repeatedly attempted to obtain more information on Brown but were denied by the King County Sheriff John Urquhart's office.
Urquhart told Seattle radio station KIROon Thursday that the investigator never identified himself as being part of the league.
" NFL, National Football League,' he could have [said] any of that," Urquhart said. "Robert Agnew, Comcast.net, post office box in Woodinville. We had no idea who this yokel is."
The King County Sheriff's Office investigated Brown -- a former Seattle Seahawks player -- for domestic violence in 2015. The misdemeanor charge was dropped within days of Brown's arrest in Woodinville, Washington, last May.
"Nowhere on the request does he say that [Agnew] works for the NFL and so, we don't know that it's the NFL and we're not gonna give it out anyway, so we denied it," Urquhart said Thursday.
In one of Brown's journal entries obtained as part of the investigation, he wrote, "I have abused my wife."
The sheriff's office released more than 150 pages of documents on Wednesday that appear to show evidence of Brown's physical and verbal abuse of his ex-wife on more than 20 occasions.
Natalie Ravitz, the NFL's senior vice president of communications, posted in a series of tweets Friday morning that the league made multiple requests to the King County Sheriff's Office in 2015 for information regarding Brown's arrest. Ravitz also tweeted photos of two documents -- the league's public information request and a police report -- which she said proves that the NFL attempted to gather the information.
In an interview with BBC Sport, NFL commissioner Roger Goodell reiterated that the league "asked repeatedly" for more information on Brown and wasn't able to gain access to it.
"We take this issue incredibly seriously," he told BBC Sport. "This is something we've been working on with policy changes, to educating our players to make sure they understand how they deal with issues with their family, give them resources to be able to deal with this. But when it happens we're not going to tolerate it. So we have some new information here, we'll evaluate that in the context of our policy, and we'll take it from there."
Brown received a one-game suspension from the league earlier this season, but the NFL now plans to review the new information.
Asked whether Brown can expect further punishment, Goodell told BBC Sport, "The policy the way it's structured right now is for an incident. And it can be reduced or increased on the basis that first offense is six games, second offence is banishment. So it's the only policy, the only issue where we have that type of penalty."
Urquhart said other investigators identified themselves as NFL representatives but they were denied since it was an open investigation.Urquhart said if the league had gone through proper public disclosure channels, the request would have gone straight to him, and hewould have been able to help out more.
"I would have said exactly the same thing, 'We cannot release the case file,'"he said. "But since this is a hot-button item in the NFL, since it's the NFL, we probably would have told them orally a little bit more about what we had.I don't like to get pushed around by a bully."
The league released the following full statement Thursday, regarding Brown's case:
"NFL investigators made repeated attempts -- both orally and in writing -- to obtain any and all evidence and relevant information in this case from the King County Sheriff's Office. Each of those requests was denied and the Sheriff's Office declined to provide any of the requested information, which ultimately limited our ability to fully investigate this matter. We concluded our own investigation, more than a year after the initial incident, based on the facts and evidence available to us at the time and after making exhaustive attempts to obtain information in a timely fashion. It is unfortunate that we did not have the benefit or knowledge of these materials at the time.
"In light of the release of these documents yesterday, we will thoroughly review the additional information and determine next steps in the context of the NFL Personal Conduct Policy. We will not be making any comments on potential discipline until that time."
The Giants announced Thursday that Brown will not travel with the team to London for Sunday's game against the Los Angeles Rams. The team said it will review the newly released documents from the Brown investigation and "revisit" the issue when the team returns from London.
NFL, Sheriff's office disagree on Brown records
The NFL says it made repeated attempts to obtain evidence and more information in Josh Brown's domestic abuse case but was denied by the King County Sheriff's Office. Sheriff John Urquhart disputes that claim.