"The Fourth Amendment to the U.S. Constitution prohibits unreasonable search and seizure. Law enforcement needs a search warrant to enter your house and seize letters from your filing cabinet — the same protections should apply to email and online documents," said Chris Gaither, a spokesman for Google.
Christina Pearson, a spokeswoman for Microsoft, which owns Hotmail, said the company requires a warrant before providing customer content to police unless there is a risk to life or serious injury.
Lauren Armstrong, a Yahoo spokeswoman, said the company requires a warrant before disclosing emails or instant messages.
Frederic Wolens, a public policy manager for Facebook, pointed to the site's webpage for law enforcement, which states that a search warrant is required to access stored data including "messages, photos, videos, wall posts, and location information."
All of the companies said their policies are based on United States v. Warshak, a 2010 federal appeals court ruling that found that police violated a man's constitutional rights by reading his emails without a warrant.
Traditionally, the courts have ruled that people have limited privacy rights over information they share with third-parties, like email providers or social networks.
The Supreme Court has not addressed the Warshak case, and the decision does not bind police outside of the court's jurisdiction in Tennessee, Ohio, Michigan and Kentucky.
But the four companies said that after the decision, they began requiring police to have a warrant before turning over email information for any of their customers.
The companies all said they adopted their policies in either late 2010 or early 2011.
Chris Calabrese, a lobbyist for the American Civil Liberties Union, said he's glad that some companies have stronger privacy policies than the law requires, but he said it doesn't give him much comfort.
He worried that if police pressed for the information, they could force the companies to turn it over. He also said smaller companies may not employ such stringent privacy safeguards.
"We really need this to be the legal standard," he argued.
The ACLU, along with Google, Microsoft, Yahoo and Facebook, are all lobbying Congress to revise ECPA, according to their most recent lobbying disclosure reports.
Leahy said earlier this month that updating the law is one of his top priorities in the new Congress and was one of the reasons he decided to stay on as chairman of the Judiciary Committee.
Sen. Chuck Grassley (R-Iowa), the panel's ranking Republican, said last November that he agrees Congress should re-examine ECPA, but he expressed concern that a warrant requirement could hamper critical government investigations.
He said Justice Department officials told him that Leahy's bill could hurt their ability to bring civil cases.
"While I agree with the business and privacy groups that there is merit to harmonizing the legal requirements for obtaining emails with a search warrant, we would be abdicating our duty if we did not examine the concerns raised by federal, state, and local law enforcement," Grassley said at a committee business meeting.