So called investigator Jack Davenport called looking for a relative, gave a case #, and said relative had a civil complaint filed against them; also said he worked for a litigation consulting service of attorneys, and relative was being sued. Would not give attorney or company name; this was the call back number to investigator. Why couldn't investigator find this person hisself? Relative's been in same place & number for over 20 years!

this is standard verbage for this scam.  
all illegal even for LEGITIMATE debt collection companies.  Some legitimate agencies do this in violation of FDCPA too
Claim to be invesitgators
Threaten immediate arrest if not paid by credit card at that time
Refuse to provide any information regarding the alleged debt (creditor name, account numbers etc)
Refuse to send anything in writing until after you have paid them
Contacting family, friends and employer and tell them anything about the alleged debt.

Fake Debt Collectors ? Terrorizing Consumers

http://abcnews.go.com/Business/story?id=5621205&page=1

Do not give this person or anyone else any information.The same thing happened to me 5 years ago from the infamous Inv. Ramsey. He will threaten to have you arrested if you do not give him access to your bank account. My so called debt was fraudulant and according to him was 20 years ago.He was trying to secure my bank account and s.s. number.He said you will pay me or the sheriff would be arresting me on my front porch tomorrow.It didnt and will not happen.He will refuse a call back number or employee no.Turn him into the authorities if he refuses to stop.He is calling a relative of mine again and so I will be turning him in again.

This guy is a joke do not gave any info called the number 202-386-6281 as you know he has made different calls posing as an official I have called the above number and this is a resident number if you do your searches on this number you can get his Name and Address some searches will show that the address is private and some will give all for a price of 9.95. be careful not to give out any personal info. If decide to call this person use the *67 wait for a double tone then dial the complete number that way he will not be able to identify you and a call back. Beside have fun with him on the call give a joke on him.

Caller identified himself as Investigator Craig Ramsey, looking for a relative Yvonne_________.  Gave case number MBB-3502 to reference when we returned a call to the above number, and stated that this was a serious legal matter.

Caller Identified himself as special agent Donald Jasper and that I was a respondant to a law suit and that it would be filed against me in my county of residence and that my I would need to appear with my attorney in reference to case number BTA-1556.  The call came in to a relative's phone number.  If he is a speal agent with Investigative power's he would have called me.