Brother and sister charged after an explosive device was found outside MacDill Air Force Base

TAMPA, Fla. - A Florida man accused of planting an explosive device outside MacDill Air Force Base has fled to China, while his sister is now in custody after both were charged in connection with the incident, federal officials announced Wednesday.

According to the Federal Bureau of Investigation, Alen Zheng, 20, faces multiple federal charges, including attempting to damage government property, unlawfully making a destructive device and possessing an unregistered destructive device. His sister, Ann Mary Zheng, 27, was booked on charges of witness tampering and acting as an accessory after the fact.

What we know:

The U.S. Attorney's Office of the Middle District of Florida said the investigation began on March 10, when a 911 call warned of a bomb near the base’s visitor center. At the time, no device had been located.

"It was very short and very cryptic, but he said that a bomb had been placed on MacDill Air Force Base," U.S. Attorney of the Middle District of Florida Gregory Kehoe said.

Kehoe says the device was found in a secluded area.

"You have something in a secluded location and hidden in the visitors center, and it just took a little bit more time to get there," Kehoe said. "If you're going to examine our more sensitive assets on the MacDill Air Force Base, not that the visitors center isn't an important asset, but obviously, Central Command is there."

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Six days later, on March 16, an Air Force service member discovered a hidden explosive device near the visitor center. Investigators later determined it was an improvised explosive device capable of causing serious harm or death.

Dig deeper:

"The bomb is placed on the 10th, they left on the 12th, but on the 11th, they attempted to destroy evidence by selling the automobile involved to CarMax," Kehoe said. "They sold it the day after they placed the bomb."

"Why it didn't detonate at the time is a question we'd like to ask Mr. Zheng," Kehoe said.

"Our last notification is that he is in the Peoples' Republic of China, as we speak," Kehoe said.

According to federal prosecutors, Alen Zheng purchased the phone used to make the threat at a Best Buy, and surveillance video captured him making the purchase and using a vehicle later tied to the case.

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Investigators say Alen Zheng and Ann Mary Zheng quickly made plans to leave the country. The day after the bomb threat, the pair allegedly sold the vehicle used to transport the device to CarMax and booked flights to China.

They both left for China by March 12, officials said.

Ann Mary Zheng later returned to the U.S. and was stopped in Detroit, Mich., on March 17. Authorities allege she helped her brother escape and tried to get rid of evidence tied to the investigation.

During a search of Alen Zheng's Land O'Lakes home, investigators said they found items consistent with those used to build the explosive device. Traces of explosive residue were also reportedly found in the vehicle the pair sold.

Authorities also confirmed no charges have been filed against the suspects’ mother, though she is currently in custody for deportation proceedings.

In a separate federal case, Jonathan Elder, 35, was arrested in Pinellas County earlier this week for allegedly making threats against MacDill AFB. 

What we don't know:

Officials say the Zhengs' motive remains unknown, and it is unclear why the device did not detonate. 

What's next:

"We are exploring every avenue to get [Alen] Zheng back to the United States," U.S. Attorney Gregory Kehoe said during a press conference Thursday.

The Associated Press contributed to this report.