The Brown University gunman was able to secretly case the school for weeks as part of his meticulously plan, which included him dodging license-plate readers and using a phone gizmo to hide his location.

A custodian at the Ivy League campus told authorities during their manhunt for the shooter this week that he had noticed a “suspicious person” lurking around the school’s engineering building Nov. 28 and Dec. 1, according to a 35-page affidavit.

A later review of surveillance video from the area confirmed soon-to-be killer Claudio Neves Valente, 48, was in the vicinity of the building Dec. 1 — fewer than two weeks before the former Brown student fatally shot two students and wound nine others there.  

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

Two weeks later, on Saturday, surveillance footage captured Neves Valente circling the building before he ultimately went inside and opened fire.

On Monday, Neves Valente then murdered MIT professor Nuno Loureiro about 50 miles away before killing himself, likely Tuesday, with his body found in a storage unit  Thursday night.

Neves Valente evaded capture for days by using European SIM cards and a type of phone that makes it harder to track someone or ping a location, according to prosecutors.

The killer was “sophisticated in hiding his tracks,’’ US Attorney for the District of Massachusetts Leah Foley said during a Thursday press conference

“He was using a phone that was obfuscating ability to track it, and he was using not credit cards that were tied to his name,” she said.

Neves Valente switched the Florida license plate on his rental Nissan with an unregistered Maine plate after the Brown shooting in a further bid to mix up cops, according to prosecutors.

He also would take back streets to avoid license-plate readers, authorities said.

“He was communicating with apps. He was utilizing certain ways to drive and picking certain neighborhoods to drive around. He wasn’t picking main streets,” Providence Police Chief Oscar L. Perez Jr. said.

The cold-blooded killer also attempted to change the drop-off for his gray Nissan Sentra car rental from Boston to Hartford, Ct., in another possible ploy to throw off authorities.

Investigators were panicked that he’d eventually try to flee the country through Boston’s Logan Airport, sources told CNN.

It didn’t hurt that Brown’s security around the targeted building was allegedly woefully lacking — with only cameras at its front part.

But despite that and Neves Valente’s careful plotting, he apparently knew his days were numbered and killed himself in the New Hampshire storage unit before cops could get him.

Authorities received their  big break in the case when a homeless man, only identified as John, came forward this week  to tell cops he had seen the perp acting suspiciously in areas surrounding the school before the mass shooting.

John, who had apparently been sleeping in the building’s basement, said he first encountered the shooter in a bathroom before having a strange “cat and mouse” interaction with him as he followed him around the neighborhood.

The homeless man later provided details on the suspect’s car that authorities say ultimately led them to Neves Valente.

The custodian came forward after John’s report surfaced.