Long, who opened fire at the country music bar in Thousand Oaks on Wednesday, was seen by a mental health specialist in April who anxious the 28-year-old Marine veteran might have post-traumatic stress disorder.
Police found patrons holed up in the attic and knocked on the door, telling them it was OK to come out.
Dean estimated 10 to 15 people, including one with a gunshot wound, had gone to hospitals.
Ms Teylor Whittler managed to escape the carnage unhurt.
Sheriff Dean also revealed Long was known to the department following a number of "minor events" in recent years.
He said he has no doubt Helus' tactics at the scene "were sound when he went in".
The gunman used a Glock 21.45-caliber handgun created to hold 10 rounds plus one in the chamber, but the gun had an extended magazine, the sheriff said. He said he thought their injuries were minor, and that majority were likely injured as they escaped, some by breaking windows.
The sheriff said a mental health crisis team was called at that time and concluded that Long did not need to be taken into custody.
"I've learned it doesn't matter what community you're in", Dean said.
"Ron was a hardworking, dedicated sheriff sergeant".
In the Facebook posting, the source confirms the writer said, "I hope they call me insane. wouldn't that just be a big ball of irony?"
He said he and others smashed their way out of the bar onto a balcony and then jumped down to safety. She was appalled that such bloodshed had come to her community.
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Upon hearing the gunshots, he immediately grabbed people around him and brought them to the ground for cover.
A man holds his head in his hands outside the Borderline Bar and Grill in Thousand Oaks, following the shooting.
"He was a hero before and he remains a hero now", said Ventura County Sheriff Union President Scott Peterson, who went to the academy with Helus some 30 years ago.
"It's a gun culture", he said.
"When (he) started to reload, that's when we got people out of there and I didn't look back", Wennerstron said.
An American flag hung on the garage at Long's home on Thursday as law enforcement gathered outside.
Ventura County Geoff Dean addresses the media about the mass shooting.
The hooded killer used a smoke bomb and a handgun, sending hundreds of people fleeing in terror.
At the White House, Trump touted his efforts to fund work on PTSD among veterans.
Telemachus was at the Route 91 Harvest festival in Vegas last October when gunman Stephen Paddock began shooting revelers from the window of the Mandalay Bay Hotel, where he was saying.
Helus was a 29-year veteran of the force with a wife and son and planned to retire in the coming year, said the sheriff, who choked back tears several times as he talked about the sergeant who was also his longtime friend.