Joseph's mother said he gave the plates to a heavenly messenger.
Oliver wrote to David Whitmer who could not leave immediately because he had to plow and fertilize his farm. He woke up to find fields plowed and his sister said three men had fertilized them. His father said the divine intervention showed he better go to Pennsylvania.
David Whitmer's account of the trip:
I was a little over two and a half days going, and traveled over 40 miles the first day, and met them [Joseph and Oliver] on the third day....
Oliver told me, they knew just when I started, where I put up at night and even the name on the sign board of the hotel where I stayed each night, for he had asked Joseph to look in the Seer stone, that he did so and told him all these particulars of my journey. Oliver asked me when I first met them, when I left home, where I stayed on the road, and the names of the persons at Hotel
I could not tell the names; but as we returned I pointed out the several houses where I had stopped, when he took out his book and found them to correspond even to the names on the sign boards, all of which he had written before we met. As had been told to him by the Prophet, and which agreed in every particular.
Cook, Whitmer Interviews, p. 123, Orson Pratt/Joseph F. Smith Interview, 7-8 September 1878, Richmond, Missouri. Joseph F. Smith Diary, LDS Church Archives
When I was returning to Fayette, with Joseph and Oliver, all of us riding in the wagon, Oliver and I on an old-fashioned, wooden, spring seat and Joseph behind us, we were suddenly approached by a very pleasant, nice looking old man in a clear open place, who saluted us with, "Good morning, it is very warm," at the same instant wiping his face or forehead with his hand.
We returned the salutation, and by a sign from Joseph I invited him to ride if he was going our way, but he said very pleasantly, "No, I am going to Cumorah."
This was something new to me, I did not know what Cumorah meant, and as I looked enquiringly at Joseph, the old man instantly disappeared, so that I did not see him again.
Joseph F. Smith: “Did you notice his appearance?”
David Whitmer: “I should think I did. He was, I should think, about 5 feet 9 or 10 inches tall and heavy set, about such a man as James Vancleave there, but heavier. His face was as large. He was dressed in a suit of brown, woolen clothes; his hair and beard were white, about like Brother Pratt's, but his beard was not so heavy. I also remember that he had a sort of knapsack on his back, and something was in it which was shaped like a book. It was the messenger who had the plates.
Orson Pratt/Joseph F. Smith Interview (1) 7-8 September 1878, Richmond, Missouri. Joseph F. Smith Diary, LDS Church Archives
Edward Stevenson also told about the same event:
He [David Whitmer] also relates an little very interesting incident that occurred in June 1829.
David, Oliver, & Joseph were riding from Harmony Pa.—the 2 former in front & Joseph back sitting in the bed on hay or straw David had been down with his team over 100 miles to fetch Joseph up to his mothers to translate the Book of Mormon about 2 ½ days drive.
While thus riding an aged looking old man came walking along putting his hand on the wagon bed, he had on his back a knapsack & the strap crossed on his breast he took his handkerchief and wiped his face to remove the sweat as it seemed to them David who was driving his team said to the man will you get up and ride.
No said he I am only going over to Comorah & suddenly disappeared they stopped the team at the sudden disappearance of the fine looking stranger he says that they all felt so strangely—that they asked the Prophet to enquire of the Lord who this stranger was.
Soon David said they turned around & Joseph looked pale almost transparent & said that was one of the Nephites and he had the plates of the Book of Mormon in the knapsack.”
After their arrival home they felt the influence of this same personage around them for he said there was a Heavenly feeling with this Nephite.
All felt very strange concerning this personage and the Prophet was besought to inquire of the Lord concerning him. Shortly afterwards, David relates, the Prophet looked very white but with a heavenly appearance and said their visitor was one of the three Nephites to whom the Savior gave the promise of life on earth until He should come in power. After arriving home, David again saw this personage, and mother Whitmer, who was very kind to Joseph Smith, is said to have seen not only this Nephite, but to have also been shown by him the sealed and unsealed parts of the plates from which the Book of Mormon was translated.
Edward Stevenson, “Visit,” Instructor 22 (1887):55; Edward Stevenson Interview 9 February 1886, Richmond, Missouri; Diary of Edward Stevenson, LDS Church Archives, available at https://catalog.churchofjesuschrist.org/assets/82199881-7613-43e6-a79e-b72609d95b23/0/22