investigation was inconclusive

When you submit the account recovery form    Then after you submit the form and if you get an auto reply email that said investigation was  inconclusive that mean that you did not submit enought information on the form

for gmail to verify the account ownership. 

 

They don't collect a lot of information from you when you sign up  for Gmail, then without the information requested about your account, they have no way to verify that the person making the request is

the  account owner. If you can recall more details about your account, you can submit the  form again. Every bit of information helps.

Because the return of your account depends on the strength of your responses,

Gmail encourage you to answer each question as thoroughly and accurately as possible. 

Please note that without accurate responses, 

Gmail can't guarantee the return of your account.