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The Broward County Sheriff's Office is investigating why three of its deputies apparently remained outside the Florida high school where 17 people were killed this month, Sheriff Scott Israel said Sunday.

Two law enforcement sources told NBC News that the deputies didn't enter Marjory Stoneman Douglas High School in Parkland during one of the nation's deadliest school shootings on Feb. 14. Israel told NBC News on Sunday that the department's internal affairs division was trying to determine whether they stayed outside when they shouldn't have.

The acknowledgement came three days after Israel announced the resignation of another deputy, Scot Peterson, who'd been assigned to the high school but remained outside during the shooting, and as the sheriff's response to the shooting came under increasing scrutiny.

Gov. Rick Scott asked the state's law enforcement agency to investigate that response on Sunday.

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Meanwhile, in a letter signed by 73 state lawmakers and released Sunday, Florida House Speaker Richard Corcoran asked Scott, a Republican, to suspend Israel, a Democrat, immediately over the department's alleged failures to appropriately handle "repeated warning signs" from the man who's been charged with the shooting, Nikolas Cruz, 19.

Today I sent the following letter to @FLGovScott Asking that he suspend Broward County Sheriff Scott Israel for incompetence and dereliction of duty. I was honored to be joined by 73 Republican colleagues. You can read the letter attached here. pic.twitter.com/exZVwGQ3DR — Richard Corcoran (@richardcorcoran) February 25, 2018

During one report to the sheriff's office in 2016, for example, a caller said Cruz planned to shoot up a school and had posted photos on Instagram in which he was holding a gun. Last November, another caller said Cruz was collecting guns and knives and described him as a school shooter in the making.

The sheriff's office didn't immediately respond to a request for comment on Corcoran's letter. Earlier, Israel dismissed an open letter to Scott from state Rep. Bill Hager, who backed Corcoran and called for the sheriff's ouster.

In a response to Scott (PDF), Israel disputed Hager's "reckless letter," saying it was filled with "factual errors, unsupported gossip and falsehoods." In an interview, Israel said he believed the effort against him was "politically motivated."

The FBI has also acknowledged that it didn't investigate a message left on a tip line about Cruz's "gun ownership, desire to kill people, erratic behavior, and disturbing social media posts."

Israel said investigators were still trying to figure out whether the other deputies who reportedly remained outside Marjory Stoneman on Feb. 14 should have pursued Cruz.

He said Coral Springs Police Chief Tony Pustizzi told a sheriff's official that as his officers approached the high school on Feb. 14, they saw two or three deputies outside the school. Investigators plan to take statements from those officers and review possible surveillance video.

"At this point, I have no independent knowledge from video or personal knowledge that any Broward sheriff's deputy did not go in when they should have," he said.

Coral Springs police have declined to comment on the reports, saying in a statement: "Any actions or inactions that negatively affected the response will be investigated thoroughly, and the information will be released officially from the proper agency spokesperson."

Marjory Stoneman held a voluntary orientation on Sunday afternoon as the school prepared for a phased-in reopening this week. Staff members would return Monday and Tuesday, Broward County Public Schools said in a statement, while students would return Wednesday.