A brother and sister have been indicted after authorities say one of them planted a potentially deadly explosive device outside MacDill Air Force Base in Florida — then fled to China, while their mother is now in ICE custody after allegedly telling investigators her son confessed to the plot.

The FBI said Alen Zheng, who is believed to have planted the device, is currently in China. He is facing charges of attempted damage to government property by fire or explosion, unlawful making of a destructive device and possession of an unregistered destructive device, which carry a potential sentence of up to 40 years in prison.

FBI Tampa also arrested his sister, Ann Mary Zheng, who is charged with accessory after the fact and tampering with evidence, facing up to 30 years in prison.

She is accused of hiding or damaging a 2010 Mercedes-Benz to prevent its use in legal proceedings, court documents show. 

Prosecutors allege that on March 11, the day after the bomb was planted, the siblings attempted to cover their tracks by selling the vehicle to car dealer CarMax. Despite being vacuumed and cleaned, investigators later discovered trace explosive residue inside the vehicle.

The indictments were unsealed Thursday morning, and U.S. Attorney for the Middle District of Florida Greg Kehoe detailed the timeline of the shocking incident during a news conference Thursday afternoon.

On March 10, Alen Zheng allegedly planted an improvised explosive device (IED) in a secluded location outside the base's visitor center. 

Minutes later, officials claim he placed a cryptic 911 call stating a bomb had been planted, but refused to provide the exact location. 

The device, which officials noted was potentially "very deadly," failed to detonate.

On March 16, an Air Force airman discovered the device outside the MacDill visitor center, six days after it was planted. 

To preserve evidence, the device was secured and flown via a borrowed Pasco County Sheriff's Office helicopter to an FBI explosives lab in Huntsville, Alabama, according to authorities.

"Anytime somebody puts an IED together — and I spent a lot of time in Iraq and I saw a lot of IEDs — there always is a level of professionalism," Kehoe said Thursday. "And quite a bit of professionalism when they end up being deadly. … [The explosive] certainly could have caused significant damage to people that were in the range."

Investigators quickly traced the 911 call's digital footprint to a burner phone Zheng purchased at Best Buy, corroborating the purchase with store security footage, officials said. 

A subsequent search of his home uncovered IED components consistent with the bomb found at the base.

The siblings fled to the People's Republic of China on March 12, just two days after the incident, according to authorities. Ann Mary was apprehended after returning to the U.S. via a Detroit airport on March 17. 

The pair's mother, who admitted to authorities that her son confessed to the plot, is currently in Immigration and Customs Enforcement (ICE) custody pending deportation for a visa overstay, but has not been criminally charged as of Thursday afternoon.

Officials have not yet publicly confirmed a motive or ties to the Chinese government. However, Kehoe said the suspects "obviously felt quite strongly about something or anything that the United States government was doing."

"Today’s indictments are the result of tremendous investigative work from our FBI teams and great coordination from our state, local, and federal partners across the board," FBI Director Kash Patel said in a statement to Fox News Digital.

"No one who targets our brave service members and military facilities will ever get away with it — and this FBI will pursue all those responsible for the incident at MacDill Air Force Base to the ends of the earth," he added.

Officials also announced a third indictment against Jonathan James Elder for an unrelated copycat threat. 

On March 18, Elder allegedly called the base making explicit threats and taunting officials about the "surprise at MacDill Visitor Center." 

After being tracked via cell phone and Facebook data, Elder was arrested at a care facility, and faces up to 10 years in prison for making a threat of an explosive.

Kehoe delivered a stern warning to anyone considering similar actions.

"If you harm somebody, if you threaten to harm somebody, or if you decide that you are going to get on the telephone and you're going to telephone a threat to someplace like MacDill Air Force Base, … you will be charged by this office," he said. "We are simply not going to tolerate this type of conduct here in the Middle District of Florida."

MacDill Air Force Base houses the headquarters of U.S. Central Command (CENTCOM), which is currently handling Operation Epic Fury against Iran. 

The base also houses U.S. Special Operations Command (USSOCOM), which oversees all special operations forces across the Department of War.

This is a developing story. Please check back for updates.