The San Francisco Police Department's bomb squad was sent to investigate a suspicious bottle and a second one left in front of two homes on the 600 block of 20th Avenue and Cabrillo.
About 20 people were evacuated.
"My husband knocked on my door because I work at night and he said 'we have to evacuate.' I said I'm not going anywhere and he said no, there is a bomb squad out there," said neighbor Debbie Garabato.
Once police knew it wasn't an explosive, investigators tested the liquid said to be a flammable substance.
"There is an investigation underway. The arson investigators have rolled out on this so I'm not going to give you any background on the package themselves," said Capt. Richard Correa from the San Francisco Police Department.
Police did confirm under each bottle there was a hand written letter, apparently threatening the elderly people who live in both homes.
"At this point in time I can't go into what the note said, but it was threatening in nature," said Sgt. Wilfred Williams from the San Francisco Police Department.
This is not the first time these two homes have been targeted. In fact, neighbors say both house have had their share of graffiti painted on them. No other home in the neighborhood -- just these two.
The owner of the house confirmed someone had painted graffiti on his garage door several times. Neighbors think it may be as simple as a parking issue.
"I don't know, maybe people park in front of the garage, they call the cops and have their cars towed away. It's just random, a random pattern of paint," said neighbor Albert Louie.
He told ABC7 it's happened so many times, his son installed surveillance cameras outside the home.
Police took the surveillance tape with them, hoping it will finally help them solve this case.