Fourth:
Physical and commercial tracing.
Candy, packaging, handkerchief and store witnesses.
Fifth:
Handwriting.
The package note and earlier letters tied the anonymous communications together.
None alone is overwhelming.
Together they form a narrative.
The Oldest Investigative Method
The prompt asks what finally allowed investigators to close in despite the postal trail.
The answer is not one exotic forensic invention.
It is recognition.
People.
Someone recognized handwriting.
Someone remembered a customer.
Someone preserved letters.
Someone recognized a retail tag.
Someone remembered selling arsenic.
Someone remembered a strange conversation.
The killer used one of America's most sophisticated national systems of the era, the postal network, to create distance.
Detectives defeated that system using human memory and paper records.
Technology carried the crime.
Ordinary people reconstructed it.
The Father as Detective
John B. Penington deserves particular attention.
He was not the official detective who arrested Cordelia.
But his actions were fundamental.
He distrusted the circumstances.
He preserved correspondence.
He compared the handwriting.
He pressed the idea that the deaths were not accidental.
He helped prevent two daughters' deaths from being absorbed into a medical explanation.
Without that persistence, the case could have taken a very different path.
Many historic murders survive in records because one relative refuses the easy answer.
This was one of them.
What If the Candy Had Been Thrown Away?
Suppose the surviving chocolates were discarded immediately after the illness.
Doctors might still suspect poisoning.
But proving arsenic entered through the candy would become harder.
Suppose Penington destroyed the anonymous letters.
Handwriting evidence weakens.
Suppose the handkerchief had no tag.
One retail trail disappears.
Suppose the druggist kept no record.
Another disappears.
Suppose candy clerks remembered nothing.
Another disappears.
The case was not solved by one mistake.
It was solved because the sender made enough small mistakes that at least some survived.
Distance Did Not Erase Identity
This is the fundamental mistake behind remote murder.
Distance feels like anonymity.
It is not.
Objects carry histories.
Paper comes from somewhere.
Handwriting belongs to someone.
Poison is purchased.
Boxes are sold.
People remember.
The farther the object travels, the more steps may actually exist to reconstruct.
The murderer sees distance as empty space.
The detective sees a chain of custody waiting to be rebuilt.
The Package's Biography
Consider the poisoned parcel as though it were a person.
It has a birth.
San Francisco.
It has components.
Candy.
Handkerchief.
Note.
Wrapping.
It has a transformation.
Arsenic is introduced.
It receives an identity.
Mary Elizabeth Dunning's address.
It enters transportation.
Postal station.
Sorting.
Rail.
East.
It arrives.
Dover.
It enters a household.
The chocolates are consumed.
People become ill.
Then investigators take the object apart and ask each component where it came from.
The package's biography becomes the murder investigation.
No Need to Chase the Killer Across Delaware
The sender's greatest advantage became the strongest clue.
California.
Once Dunning named Cordelia, police knew where to look.
Had the killer traveled secretly to Delaware and purchased everything locally, the investigation might have been broader.
By sending all components from the West Coast, the sender created a geographic signature.
Everything pointed back to the same city.
The postmark narrowed the world.
Why "Mrs. C." Was a Mistake
A completely anonymous box might have made Mary Elizabeth suspicious.
So the sender needed familiarity.
But familiarity requires information.
The phrase "yourself and baby" demonstrated personal knowledge.
The initial C suggested a woman the victim might know.
Those details increased trust.
After the murder, they increased investigative specificity.
Every detail that makes a deception believable can later identify the deceiver.
The Poisoner's Paradox
To kill Mary Elizabeth remotely, the sender had to understand her.
Her address.
Her family.
Her sweet tooth.
Her willingness to accept a gift.
Her daughter.
That intimate knowledge made the attack effective.
But intimate knowledge also narrowed the suspect pool.
A random stranger would not know those things.
The perfect targeted murder contains the seeds of its own solution.
The Affair Was Supposed to Stay Private
If Cordelia was responsible, the motive came from a relationship that both participants had reason to keep discreet.
Then the murder investigation dragged that relationship into newspapers across America.
Letters.
Affairs.
Other women.
Hotel arrangements.
Financial misconduct.
Nothing remained private.
The effort to preserve one relationship by eliminating a rival instead destroyed every secret surrounding it.
This is another recurring feature of murder.
The cover-up often exposes more than the original conflict ever would.
Public Opinion Versus Legal Proof
Newspapers treated Botkin as a sensation.
Some coverage was openly judgmental about her appearance, personality and femininity.
That is important context.
A woman accused of murdering a romantic rival violated Victorian expectations in ways newspapers exploited aggressively.
Readers should separate those cultural judgments from the actual evidence.
Whether Cordelia was vain, unconventional, older than Dunning or socially disapproved of has no bearing on guilt.
The meaningful evidence is:
motive,
poison,
purchases,
documents,
handwriting,
mailing,
and witness testimony.
The sensational press and the prosecution were not the same thing.
Could She Have Been Wrongly Convicted?
Any circumstantial historical case deserves the question.
There was no direct eyewitness to the poisoning.
Witness identifications had weaknesses.
Handwriting analysis is less definitive than DNA.
The drug purchase did not itself prove murder.
Dunning had relationships with other women.
The first conviction was legally reversed.