New York's top prosecutor thinks there needs to be less phone privacy if we want to "defeat terrorism."

In a stark report issued Wednesday, the Manhattan District Attorney's office makes a detailed case showing how modern day privacy tools protect criminals -- and get in the way of police investigations.

The report specifically calls out Apple and Google.

It cited 111 times in the past year when law enforcement couldn't search iPhones -- even though police had proper court-approved search warrants. Police were investigating cases of murder, child rape, sex trafficking, and robbery.

"Every time a tip leads to a phone... it may go cold," Manhattan District Attorney Cyrus Vance said in a speech at the New York Federal Reserve Bank.

And trying to show the benefits of more police power, the report noted 8 recent cases in which photos or messages found on an unlocked iPhones and other devices provided key evidence in an investigation.

One unlocked iPhone contained a video of a shooting that led to a murder conviction. On another phone, Internet browsing history and text messages helped cops catch two accused rapists now facing life in prison.

The report is part of mounting international pressure to combat the biggest challenge in law enforcement today: the inability to track terrorists who "go dark" by encrypting their communications.

But there's a key problem: Encryption is cheap, free technology. It keeps modern day email, business plans, banking and everything else private. Even if it's banned, people can find it anywhere or build their own.

During his speech, Vance blamed Apple (AAPL) and Google (GOOGL) for making the public less safe.

Last year, both smartphone makers equipped their devices with encryption by default. Apple and Google don't have the keys to decipher the contents of your phone -- even if they wanted to.

That means police cannot grab information from a suspect's device by pressuring the phone maker. Instead, police must acquire the actual phone and either unlock the phone by hacking it -- or get the person's permission.

Local police agencies all across America use special machines that hack into smartphones. But officers on NYPD's intelligence team told CNNMoney they can't always get access to those devices.

The debate over encryption is relevant now, because the Paris attackers coordinated their efforts using apps that encrypt communication and delete messages, investigators tell CNN.

Vance's proposed solution? A federal law that says companies can only sell smartphones in the United States if the maker can unlock it.

Apple and Google did not immediately respond for comment. But in the past, both companies have taken a strong stand to protect customer privacy worldwide. Their argument: The same door that lets in law enforcement can let in hackers and also put people in danger if they are living under authoritarian regimes.

In a speech at the same event Wednesday, London police commissioner Adrian Leppard raised concerns similar to Vance's issue.

"We're unable to see as much as we could before," Leppard said. Surveillance is what allowed British security to prevent 7 attacks in the last 12 months, Leppard said.

FBI director James Comey, who has made this case repeatedly for the past year, said agents often run into dead ends.

Comey said the FBI watches as the Islamic State broadcasts publicly on Twitter. He also revealed that, with "judicial process," the FBI can monitor private direct messages on Twitter. But when recruits get serious about actually committing a violent crime, the conversation moves to encrypted apps that the FBI can't spy on.

"And at that moment, the needle [in the haystack] we've been searching the entire nation to find -- and have found -- goes invisible to us. That's the 'going dark' problem," Comey said.

Comey suggested a better partnership and understanding between American technology companies and law enforcement.

"We are drifting to a place in America where lawful court orders... that are the lifeblood of our criminal work to protect children, fight gangs... to stop bad things from happening... are ineffective because devices are protected by encryption," Comey said.