Until FBI Director Christopher Wray acknowledged Friday that the bureau’s tip-line had failed to act on crucial information about Florida school shooter Nikolas Cruz more than a month before last week’s massacre, the busy call center had been operating in near-anonymity from its hub in rural West Virginia.

Nearly 4 million telephone calls and emails have poured into the so-called Public Access Line since the operations were consolidated six years ago.

The program, which grew out of the FBI’s effort to combat massive fraud spawned by Hurricane Katrina in 2005, was aimed at centralizing separate call centers that had been functioning at all 56 field offices across the country – freeing up more agents to work the streets in divisions across the country.

In wake of the tip-line's breakdown in the Cruz case, it is now the focus of an internal FBI review. Lawmakers, meanwhile, have called for congressional inquiries and President Trump offered his own rebuke in a weekend tweet, saying that the FBI was "spending too much time trying to prove Russian collusion."

Here, we break down everything you need to know about the tip-line.

How are tips routed to the FBI public access line?

Typically, calls of all kinds are received at local FBI field offices across the country where they are sorted by topic.

Those with leads on potential crimes or threats to public safety are transferred to the call center located at the FBI's offices in Clarksburg, W.Va., the same location that houses the National Instant Criminal Background Check System that reviews the backgrounds of gun purchasers.

Who receives and assesses the information?

Former Assistant FBI Director Stephen Morris, who once oversaw the FBI's West Virginia operations, said that more than 100 staffers handle the information, which is reviewed not only for the raw material being provided but also for the source's credibility.

When possible, the information is evaluated against bureau databases in an attempt to further verify the material.

When is the tip referred for additional investigation?

The process can be largely "subjective," Morris said, as analysts assess the credibility of the source and content of the lead.

Generally, when a tip is determined to have investigative merit, a supervisor reviews the information before it is routed to the appropriate field office for further investigation.

What happened in the Cruz case?

On Jan. 5, according to FBI, the public access line received information from "a person close to Cruz" about the suspect's "gun ownership, desire to kill people, erratic behavior...and the potential of him conducting a school shooting.

"Under established protocols," the FBI said, "the information provided by the caller should have been assessed as a potential threat to life. The information should have been forwarded to FBI Miami field office, where appropriate investigative steps would have been taken... The information was not provided to the Miami field office, and no further investigation was conducted at that time."

How much information is not deemed worthy of additional investigation?

Nearly 98 percent – or 1.4 million – reports to the FBI's tip-line in 2017 did not warrant follow-up investigation, according to a report by the bureau's Criminal Justice Information Services Division.

Morris, the former assistant FBI director, said it is not uncommon for analysts to be fielding calls that have no connection to threats or potential criminal activity.

"There are some people who call in to ask why they haven't gotten their social security check. Some callers are abusive and analysts have to sort through that to determine if there is important information there. It's not an easy job."