A case being investigated by Ottawa police's sexual assault and child abuse unit was delayed until police could get permission from the Afghan government to speak to the suspect, the teenage son of a high-ranking diplomat​, CBC News has learned.

Under international protocol, Global Affairs Canada had to seek a waiver of diplomatic immunity from Afghanistan's Ministry of Foreign Affairs in Kabul to allow Ottawa police to question the diplomat's son.

Diplomatic immunity is not usually extended to family members unless the representative is from the senior ranks.

This permission through international channels was recently granted. The diplomat's son has obtained a prominent criminal defence lawyer but has yet to be interviewed by police.

The complainant is a Canadian citizen and a high school student, sources tell CBC News. The incident was first reported in an online Afghan news service.

2nd waiver would be needed for arrest

If charges are warranted after an investigation, police would once again have to ask Global Affairs Canada to intervene to request a second waiver from Kabul to make an arrest.

Ottawa police said they do not comment on open investigations.

CBC News has requested comment from the Embassy of the Islamic Republic of Afghanistan, but has yet to receive a response.

The 1961 Vienna Convention outlines the rules of diplomatic law and prevents host countries from using their laws to pressure foreign representatives. The convention requires diplomats to obey the laws of their host country, but the only punishment permitted is expulsion unless immunity is waived.

The most controversial use of diplomatic immunity to avoid prosecution involves the 2001 death of Ottawa lawyer Catherine MacLean.

MacLean was killed on Jan. 27, 2001, when she was struck by a vehicle driven by a drunk Russian diplomat.

Andrei Knyazev had claimed diplomatic immunity after the collision and returned to Moscow within days of the incident.

He was later found guilty of involuntary manslaughter in Moscow.

With files from CBC's Evan Dyer