question:  Why does Stevenson begin to suspect a connection between the crimes?
context: There were Miss English and Miss Philicoff, the girls in the gilded teller cages. There was Mrs. Elizabeth Clayhorn, depositing her husband's pay check in their joint checking account, and with her was her ten-year-old son Edward (Eddie) Clayhorn, Junior. There was Mrs. Dolly Daniels, withdrawing money from her savings account again. One of the others took the pistol away from Mister Anderson, the guard, and said to him in a low voice, "Think about retirement, my friend." The third one, who carried a black satchel like a doctor's bag, walked quickly around behind the teller's counter and started filling it with money. The man who had first spoken herded the tellers, Mr. Featherhall and the customers all over against the back wall, while the second man stayed next to Mr. Anderson and the door. She fell unconscious to the floor, and all three of them ran out of the bank to the car out front, in which sat a very nervous-looking fourth man, gunning the engine. Two police cars came driving down the block and a half from the precinct house to the bank, and the car with the four robbers in it lurched away from the curb and drove straight down the street toward the police station. "What I can't figure out," said Stevenson, "is exactly what made those tires do that. "And that doesn't make sense, either," said Stevenson. "I believe so, yes," said Stevenson. On the way around, Stevenson said, "I believe you reported the car stolen almost immediately after it happened." "If I'm making just a quick stop—I never spend more than five minutes with any one customer—I always leave the keys in the car. "It was four professional criminals, I thought you knew that. Stevenson followed Hastings' pointing finger, and saw again the crudely-lettered words, "The Scorpion" burned black into the paint of the trunk lid.
output:  The nature of how the crimes ended didn't add up on their own. That, as well as the signatures, make him believe there is more.
options:  Stevenson has an overactive imagination, similar to how a previous police officer had been.  The nature of how the crimes ended didn't add up on their own. That, as well as the signatures, make him believe there is more.  Two back-to-back crimes is too suspicious.  The alibi of Higgins doesn't add up. He admits to leaving the signature, but Stevenson doesn't trust him.