If you get arrested soon, know that any cellphone you manage to smuggle into prison may not work.

The National Telecommunications and Information Administration released a report on June 15 giving new details about “micro-jamming” technology that can render a jail cell without cellphone signal. Just 20 feet outside the area, however, wireless communication signals can run as normal.

The results are “a step forward countering the security threat posed by contraband cellphones,” said Beth Williams, assistant attorney general for legal policy at the Department of Justice, in a news release. “The results indicate the potential for localized impact of this micro-jamming technology.”

The Justice Department said in a statement that the contraband cellphones have been used to “direct gang activity, run criminal enterprises, distribute child pornography, intimidate witnesses, and facilitate the commission of violent crimes.”

The test results add to a growing number of advancements in the field of signals intelligence.

In early June, the Department of Homeland Security said that it had discovered signs of surveillance devices to track cellular communications near the White House. President Trump has allegedly ignored the advice of top counter-intelligence officials and continued to use an unsecured cellphone.

Also, Russian military intelligence has implanted malware inside cellphones using the Android platform, according to a 2016 report from the firm Crowdstrike.