The average number of days for an FAI to be undertaken and concluded in 2016/17 was 406 days, however data covering the past eight years puts the longest wait at 1,586 days in 2012/13.
The Lib Dems raised concerns over “unreasonable” delays in the process following an appeal to the Lord Advocate to initiate an FAI into the deaths of M9 crash victims Lamara Bell and John Yuill in 2015.
The couple lay undiscovered in their car for three days following the accident on the motorway, despite the incident being reported to Police Scotland at the time.
The party has also previously sought assurances that deaths in prison - which are subject to FAIs - are being properly investigated.
Figures show delays of up to 1,428 days for FAIs to be concluded into deaths in custody.
Lib Dem justice spokesman Liam McArthur said: “While I understand the complexity of these investigations, in many cases families across Scotland have faced unreasonable delays in their search for answers about the fates of their loved ones.
“With respect to deaths in prison, I have already asked the Lord Advocate if he will look again at why these delays are happening and take steps to address any underlying issues, including those related to capacity within the service.
“These new figures further suggest a systemic problem. Cases that drag on or are left unanswered leave families in limbo.
“I urge the Scottish Government to ensure that all future fatal incidents are investigated comprehensively and without delay.”
Figures provided in response to parliamentary questions from the Lib Dems show the longest waits for FAIs to be concluded each year between 2010/11 to 2014/15 all exceeded 1,300 days, with the figure dropping to 979 days in 2015/16 and 695 in 2016/17.
The average wait in each year was over 600 days, dropping to an average wait of 406 days in 2016/17 and 272 days in 2017/18 so far.
For deaths in prison, the longest wait was in 2012/13, at 1,428 days. The average in 2016/17 was recorded at 373 days, and 253 days in 2017/18 so far.
A Scottish Government spokesman said: “Investigations of deaths and decisions on fatal accident inquiries are matters for the Lord Advocate acting independently.”
The Crown Office, which is headed by the Lord Advocate, said it aims to hold FAIs “as soon as practicable”.
A spokeswoman said: “Most death investigations conclude once a cause of death is known, but some cases require further investigation which may include complex technical and medical issues requiring the instruction of experts.
“Where an FAI is held, the length of time to conclude the evidence and for the sheriff’s determination to be issued also depends on a variety of factors, including the scale and complexity of the inquiry and the other demands on court and judicial time.”