In this story: 'Final note' They were going to destroy the school' School closed indefinitely RELATED STORIES, SITES

LITTLETON, Colorado (CNN) -- A possible suicide note in the Columbine High School massacre indicates the two suspects, who killed 13 people and then themselves, acted alone, CNN has learned. Also, authorities discovered a single explosive device in the school's kitchen, originally reported to be two separate propane bombs.

In another development, Harris and Klebold may have made, or tried to make, a homemade version of napalm, sources told CNN. Federal agents would only say they found a highly flammable liquid during a search of the suspects' homes.

Earlier Thursday, school officials announced that Columbine High, in suburban Denver, would be closed indefinitely.

'Final note'

Two sources told CNN that the "final note," as they called it, was written by one or both of the two outcasts who killed 12 school mates and a teacher before taking their own lives on Tuesday.

According to sources, the note said, in essence, that Harris, 18, and Klebold, 17, acted alone, had no accomplices and wanted no one else to be blamed.

"This is the way we want to go out," the note reportedly said.

The sources said the note was not found in either of the suspects' homes, but they would not say where the note was found. Jefferson County Deputy Sheriff Steve Davis said he was aware of a note found at Harris' home, but it was not immediately clear if he was referring to the same note.

'They were going to destroy the school'

The propane tank bombs were found in duffel bags hidden in the kitchen of the school cafeteria by agents of the federal Bureau of Alcohol, Tobacco and Firearms.

"These subjects were not only on a killing rampage, but they were going to destroy the school," said Jefferson County Sheriff John Stone.

The devices, however, were not armed, nor were they booby-trapped to go off, officials said.

They were about the size of propane tanks used for barbecues. Each of the bombs weighed at least 20 pounds, authorities said.

Investigators, who were inside the school collecting evidence and searching for a motive in Tuesday's gun and bomb assault, were evacuated.

On Wednesday, police said they had uncovered about 30 explosive devices at the school and elsewhere. The discoveries on Thursday gave new rise to the prospect that Harris and Klebold may have had help, a possibility authorities have said they are looking into.

As part of that probe, acquaintances, classmates and relatives of the gunmen are being interviewed.

School closed indefinitely

Overnight snow that continued into the daylight hours kept some Jefferson County schools closed Thursday. But others, except for Columbine, reopened with tightened security.

As they announced that the high school would be closed indefinitely, Jefferson County school officials were trying to arrange for other schools in the district to take in the nearly 2,000 Columbine students so they can finish the academic year.

Police describe the inside of the school as a complicated crime scene where the collection of evidence could take days, and cleanup could take weeks.

More than 20 people were wounded in the attack, and most of them remained hospitalized on Thursday, some in critical condition.

Correspondent Greg LaMotte and Martin Savidge contributed to this report.