A custodian had spotted a “suspicious person” — later confirmed to be the shooter — lurking around the school’s engineering building on both Nov. 28 and Dec. 1, according to a 35-page affidavit.

The custodian told investigators he encountered the man, who was wearing a surgical mask and was walking with a limp, during his shifts between 3 p.m. and sunset, the filing said.

Then, two weeks later, surveillance footage captured the same person circling the building just hours before he went inside and opened fire.

The custodian’s sighting came before a homeless man — only identified as John — came forward to say he, too, had seen the perp acting suspiciously in areas surrounding the school prior to the bloodshed.