The envoy, Alfred Nelson-Williams, was snatched on Friday as he was travelling by road from the Nigerian capital of Abuja north to the city of Kaduna.
"We've been informed by the Nigerian authorities that the driver of the kidnapped deputy high commissioner is being questioned by security officials," Sierra Leone Deputy Information and Communication Minister Cornelius Deveaux told reporters today.
Kidnappings are common in Nigeria, where the rich and powerful drive bulletproof cars and even hire military and police chaperones as protection from highway bandits.
Neither he nor police could confirm whether the kidnappers had demanded a ransom.
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Foreign ministry sources in Freetown told AFP that "this is the first time that a Sierra Leone diplomat has been kidnapped anywhere on posting since independence in 1961".
A source in the kidnapped diplomat's family said Saturday, "the news hit us with a bang but we are hopeful that all will be well and that God will give him divine protection".
The government of former president Goodluck Jonathan was criticised for its slow response in acknowledging the kidnapping and for its inability to find and recover the girls.
Late last month, popular musician Ado Dahiru Daukaka was also kidnapped in Nigeria's northeast Adamawa state and then freed days later.
But in the oil-producing delta region in the south, where wealthy Nigerians and expatriate workers are usually the target, abductions are often for financial gain.
Last week, three Australians, a New Zealander and a South African were kidnapped along with two Nigerians near the capital of Cross River state in the country's south.
They were released four days later, but officials did not say whether the kidnappers received any ransom.