There is not enough evidence to determine whether Svetlana Aronov, the Upper East Side woman who disappeared in March while walking her dog, died by homicide, suicide or accident, the medical examiner said yesterday.

The inconclusive report adds another layer of mystery to the death of the rare books dealer, whose disappearance captured the city's attention.

Aronov, whose body was found in the East River near Long Island City in May, drowned, said medical examiner spokeswoman Ellen Borakove. But police can't figure out how she ended up in the river and what happened to Bim, her father's dog, whom she was walking when she vanished.

"We still don't know how this happened," said Ed Hayes, the lawyer representing Aronov's husband, Alexander. "The examination rules out certain possibilities which none of us thought were very likely anyway. We never thought she used drugs; we never thought she was poisoned. It's still a terrible loss and a terrible mystery."

Aronov, 44, left her apartment with the dog, her house key and her cell phone around 2:30 p.m. on March 3, leaving her purse behind, police said. She never arrived at Kennedy Airport, where she was due to pick up her father at 5 p.m.

Bloodhounds picked up Bim's scent on the west side of York Avenue near the Aronovs' apartment on 64th Street and York but lost it at 68th Street.

At first, police investigated her disappearance as a homicide. But after her body surfaced in the river on May 6, police sources say they are more inclined to consider Aronov's death a suicide, because she had a history of depression and no defensive wounds.

But that doesn't explain why the rare book dealer would have taken the dog along, why she began dinner preparations, or why not a single witness reports seeing her fall from the FDR Drive.

Hopes that the autopsy and toxicology testing would shed light on the case were dashed yesterday when the medical examiner's report proved inconclusive.

"We do not have enough information to make a ruling," Borakove said.