the crime has been committed abroad
In cases where accused person(s) has(have) escaped from the country after committing the crime or part of the crime has been committed abroad or the witnesses and other material evidence are available in a foreign country, it may be considered necessary to conduct investigation abroad.
While informal information/material may be collected through Interpol and diplomatic channels, formal investigation to collect evidence and gather material objects/documents may be conducted through formal Letter of Request sent through a competent Court under provisions of 166-A of the Criminal Procedure Code.
The diplomatic missions which are situated in various countries abroad cannot help with such CBI Investigation except for providing information collected from informal channels like the Interpol.
For those countries who do not require a Letter of Request from the court for conducting such investigations, a notice containing relevant facts including various important facets of the investigation are to be sent to the Interpol wing of the CBI, who then continue with the investigation.
When, in cases of utmost importance and absolute necessity, officers of the CBI are sent abroad for the conduct of an investigation, it is important to keep in mind that such officers have no powers in that country.
Any action which impedes upon the law on a foreign land may lead to impeding the sovereignty of the country. In such cases, the National Central Bureau (Interpol) of that country may be requested through the Interpol Wing of CBI and the Competent Authority of the country through the diplomatic channels for taking necessary permission/approving visit of the Police Officers to the concerned country.
Any work that needs to be done out of the territory of India can only be started once such permission has been obtained from the relevant authorities.