Follow Dogmeat who’s tracking Kellog’s scent
Dogmeat will use his scent to hunt down Kellogg one clue at a time. Follow him.
Find a cigar at the first stop
When Dogmeat stops for the first time, at a pond, look for a cigar in the immediate area. Continue following Dogmeat.
Find a bloody rag at the second stop
After Dogmeat stops again, look for a bloody rag at a freeway underpass. Continue following Dogmeat.
Find another cigar at the third stop
After Dogmeat stops again, at a railroad shack, find another cigar there.
Find robot Assaultron
Find the remains of a robotic Assaultron and talk to it. (See Fallout 4: Getting a Clue Walkthrough)
Reach Fort Hagen and find another clue
Once you reach Fort Hagen, Dogmeat will lead you to another bloody rag for you to analyze.
Enter Forg Hagen
You can enter the Fort by either going through an underground parking lot or through a hatch on the roof. Be prepared for challenges ahead. (See Fallout 4 – Prologue Walkthrough)
Reach Kellogg and defeat him after talking to him
Follow the main path to reach Kellogg. After talking, be prepared to fight him.
Loot Kellogg and leave
Loot Kellogg’s for his terminal password and his Cybernetic Brain Augmenter. Use the nearby terminal to open the security doors.
Go back to Diamond City and talk to Piper
Return to Diamond City and talk to Piper at either her office or at the Valentine Detective Agency.