(CNN) In the aftermath of last week's deadly terrorist attack at two mosques in New Zealand, local authorities confirmed that personnel from the US Federal Bureau of Investigation had arrived in country to assist with their investigation.

The FBI has declined to comment on exactly how large of a footprint it has established in the small Pacific nation more than 6,000 miles from the continental United States, which illustrates both the reach and low-key international posture of the crime fighting agency.

A law enforcement source tells CNN that the bureau has also searched through the agency's intelligence holdings for any information that may be of value to investigators in New Zealand.

"It's not the FBI's style to publicize every overseas effort," said Gina Osborn, a retired FBI assistant special agent in charge who led one of the bureau's international response teams. "We respond to requests for assistance every day, but there's no reason to announce it."

While the FBI's domestic operations involving the protection of the homeland from national security and criminal threats are widely understood, its operations overseas involving personnel stationed across the world are less well known. That work is augmented by agents and analysts in the US who deploy in support of specific investigations.

"There is no typical day overseas with the FBI," said Gib Wilson, a retired legal attaché who served in Belgium, India, and the Philippines. "You're simultaneously trying to further FBI investigations, help our international partners, and work with the intelligence community to stop dangerous threats. You are truly the pivot point in so many investigations aimed at protecting innocent people around the world."

Longstanding global presence

According to the FBI, the bureau's global presence traces its roots back to the pre-CIA World War II era, when President Franklin Roosevelt ordered government agencies to assist in collecting intelligence about the Nazis. The first agent was assigned to Bogota in 1941, followed by London, Ottawa and Mexico City.

FBI data indicates the agency currently has Legal Attaché offices and sub-offices in nearly 90 countries, staffed by employees who serve multiple-year tours as liaisons covering more than 180 countries, territories and islands. These permanently-assigned representatives fall under the purview of the American ambassador appointed to a specific country and serve as members of the "country team" -- the collection of US agency representatives assigned to embassies.

"By increasing our international footprint we've been able to develop long-lasting relationships with our foreign partners," said FBI special agent in charge George Piro last year while speaking at the State Department's Foreign Press Center. "We in the FBI recognize how important it is for information sharing but also for sharing of best practices, trainings, lessons learned -- not only from our perspective but gaining that from our foreign partners."

As Piro and other experts have noted, a significant portion of the work done by agents and analysts overseas involves providing training in areas such as behavioral analysis, conducting interviews, processing crime scenes and the exploitation of digital evidence.

'Work overseas never stops'

"The FBI's capacity-building work overseas never stops," said Megan Stifel, a senior fellow at the Atlantic Council who previously served as an attorney in DOJ's National Security Division. "US law enforcement works with foreign governments well in advance of critical incidents to train them on the value of evidence, and how to effectively collect and use it to solve crime."

Once a crime or act of terrorism abroad does occur, FBI investigative assistance is frequently requested by partner nations. If an incident impacts directly on the US causing harm to an American citizen or company overseas, the bureau can open a case of its own.

For these investigations, the bureau maintains extraterritorial squads in four of its field offices -- Los Angeles, Miami, Washington and New York -- who are responsible for launching personnel overseas to investigate threats to American interests and working to prosecute suspected criminals in either US or foreign courts of law.

Although these squads are charged with enforcing US extraterritorial laws, they do not have unilateral authority abroad. "The FBI doesn't just hop on an airplane and start conducting investigations in a foreign country," said Osborn. "Rather, their every move must be coordinated in advance with the host nation."

One typical example of an FBI extraterritorial squad's work in assisting foreign partners was the agency's role in helping Indian investigators following the 2008 terrorist attacks in Mumbai, which resulted in the death of nearly 200 civilians.

In that case, investigators working to identify the origins of a terrorist cell discovered that the engine on a boat used by the attackers to land ashore in India was manufactured by Yamaha, a global company with offices in the United States. The FBI served a subpoena on the company to seek its cooperation in tracing the original point of sale of the engine, which was discovered to be in a location in Pakistan. This information was then relayed to Indian authorities.

"This example illustrates one of the many ways US law enforcement works to help solve international crime," said Bryan Schilling, a former national security attorney at the FBI who worked on the Mumbai investigation. "Using domestic resources and legal authorities, the FBI can frequently and quickly secure lead information to assist foreign law enforcement in local investigations. There are restrictions when it comes to passing information about U.S. persons, but when it comes to major incidents and transnational crime, there is frequently a US nexus the FBI can investigate."

The bureau's global work has not been without controversy, particularly when a seemingly routine offer of assistance has thrust the agency into the middle of foreign political turmoil. In 2015, the FBI was asked to help authorities in the Maldives investigate a blast aboard a boat carrying the President of the archipelago nation. Allies of the President declared the incident an assassination attempt, citing purported findings by the FBI and other international investigators that the explosion was not accidental.

The vice president and two bodyguards were arrested for their alleged role in a suspected coup plot. The FBI released a public statement on its investigation, which indicated investigators had in fact found no conclusive evidence that the explosion was caused by a bomb.

Although much about the US role in New Zealand remains shrouded in secrecy in order to protect the ongoing terrorism investigation, there appear to be similarities to the low profile manner in which the FBI participated in investigating the 2008 Mumbai attack. In that case, bureau personnel helped collect evidence, analyze intelligence, and eventually testified in an Indian court. Asked about what, if anything, bureau investigators have unearthed while providing assistance to authorities in Christchurch, an FBI spokesperson referred questions to the New Zealand police.

Josh Campbell is a CNN law enforcement analyst and a former Supervisory Special Agent with the FBI.