In an interview Friday, Chief Lanier defended her detectives saying Human Rights Watch has reached a number of conclusions without having access to all of the data.
Lanier denies cases go undocumented and to prove it, she has asked the Civil Rights Division of the Department of Justice to come in and take a look.
"Just as Human Rights Watch is trying to protect the people they interview, we have to protect the people in our investigations, and as a result, we were not able to turn over all of the documentation they need to draw conclusions," said Chief Cathy Lanier.
Chief Lanier said the Department of Justice would have unrestricted access and be able to fully see the department's actions and procedures.
In the letter, Human Rights Watch said it found:
"The hospital records more reports of sexual assaults than incidents on the (Metropolitan Police Department) website and (police reports of sexual assaults) every month."
"I've looked," said Lanier. "I know we have the documentation. The case has been documented and followed up on, but I can't turn some of those things over."
Chief Lanier said not every category of sexual assault is reflected in the website numbers.
The letter also said, "Detectives regularly treat victims in a dismissive or insensitive manner adding to their trauma."
"Between 2006 and 2008, I think some of those allegations, there maybe some truth to them, but I think we have done an awful lot of work and put an awful lot of reforms in place," Lanier said.
But the lawyer who wrote the letter, Sara Darehshori, said she is convinced the department still has a problem handling sexual assaults.
"We do believe there is a serious problem based on the information we have," said Darehshori with Human Rights Watch. "And we are open to reviewing additional information that shows that if the analysis we have undertaken and the information we've gathered is incorrect, we are open to seeing documentation we haven't been given access to despite our request over 16 months."
Darehshori said the letter to the chief was a heads up to what its investigation has uncovered and a full report will follow.
Human Rights Watch said it has agreed to hold off issuing a full report until it has seen the additional documentation.
The Department of Justice has not said publicly yet whether it will accept the chief's offer and begin a probe.
Human Rights Watch bills itself as a non-profit fact-finding organization dedicated to defending and protecting human rights.