The justices didn’t exactly agree on the rationale, but all nine ended up in the right place.

The Supreme Court on Monday unanimously ruled that the police violated the Constitution when they placed a Global Positioning System tracking device on a suspect’s car and monitored its movements for 28 days.

But the justices divided 5-to-4 on the rationale for the decision, with the majority saying that the problem was the placement of the device on private property. That ruling avoided many difficult questions, including how to treat information gathered from devices installed by the manufacturer and how to treat information held by third parties like cellphone companies.

Walter Dellinger, a lawyer for the defendant in the case and a former acting United States solicitor general, said the decision “is a signal event in Fourth Amendment history.”

In this case, the police placed a GPS tracker on a suspected drug dealer’s jeep without a warrant for four weeks, and then used the obtained information to get a conviction. Five justices — Scalia, Roberts, Kennedy, Thomas, and Sotomayor — said the device constituted a search of private property. Four justices — Alito, Ginsburg, Breyer, and Kagan — focused on reasonable expectations of privacy.

The next question is how this will apply to related technological advances, most notably the tracking of mobile phones, which would not require law enforcement to physically place anything on a person or their vehicle. On this, it appears the Supreme Court’s breakdown would be slightly different.

Justice Sotomayor joined the majority opinion, agreeing that many questions could be left for another day “because the government’s physical instruction on Jones’s jeep supplies a narrower basis for decision.”

But she seemed to leave little doubt that she would have joined Justice Alito’s analysis had the issue he addressed been the exclusive one presented in the case.

“Physical intrusion is now unnecessary to many forms of surveillance,” Justice Sotomayor wrote…. “People disclose the phone numbers that they dial or text to their cellular providers; the URLs that they visit and the e-mail addresses with which they correspond to their Internet service providers; and the books, groceries, and medications they purchase to online retailers,” she wrote. “I for one doubt that people would accept without complaint the warrantless disclosure to the government of a list of every Web site they had visited in the last week, or month, or year.”

It would appear, then, there are five votes for broader use of warrants when it comes to modern technology. That’s encouraging.

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Steve Benen

Follow Steve on Twitter @stevebenen. Steve Benen is a producer at MSNBC's The Rachel Maddow Show. He was the principal contributor to the Washington Monthly's Political Animal blog from August 2008 until January 2012.