WASHINGTON — Can you trust what a dog’s nose knows? Police do, but the Supreme Court considered Wednesday curbing the use of drug-sniffing dogs in investigations following complaints of illegal searches and insufficient proof of the dog’s reliability.

Justices seemed concerned about allowing police to bring their narcotic-detecting dogs to sniff around the outside of homes without a warrant and seemed willing to allow defense attorneys to question at trial how well drug dogs have been trained and how well they have been doing their job in the field.

“Dogs make mistakes. Dogs err,” lawyer Glen P. Gifford told the justices. “Dogs get excited and will alert to things like tennis balls in trunks or animals, that sort of thing.”

But Justice Department lawyer Joseph R. Palmore warned justices not to let the questioning of dog skills go too far, because they also are used to detect bombs, protect federal officials and in search and rescue operations. “I think it’s critical ... that the courts not constitutionalize dog training methodologies or hold mini-trials with expert witnesses on what makes for a successful dog training program,” he said.

“There are 32 K-9 teams in the field right now in New York and New Jersey looking for survivors of Hurricane Sandy,” Palmore added. “So, in situation after situation, the government has in a sense put its money where its mouth is, and it believes at an institutional level that these dogs are quite reliable.”

The arguments on Wednesday revolved around the work of Franky and Aldo, two drug-sniffing dogs used by police departments in Florida.

Franky’s case arose from the December 2006 arrest of Joelis Jardines at a Miami-area house where 179 marijuana plants were confiscated. Miami-Dade police officers obtained a search warrant after Franky detected the odor of pot from outside the front door. The trial judge agreed with Jardines’ attorney that the dog’s sniff was an unconstitutional intrusion into the home and threw out the evidence.

A Florida appeals court reversed that ruling, but the state Supreme Court sided with the original judge.

The Florida Supreme Court also threw out work done by Aldo, a drug-sniffing dog used by the Liberty County sheriff. Aldo alerted his officer to the scent of drugs used to make methamphetamine inside a truck during a 2006 traffic stop, and Clayton Harris was arrested. But two months later, Harris was stopped again. Aldo again alerted his officer to the presence of drugs, but none were found.

The Florida Supreme Court justices ruled that saying a drug dog has been trained and certified to detect narcotics is not enough to establish the dog’s reliability in court.

The state of Florida appealed both cases to the Supreme Court.

Harris’ lawyer Gifford asked the court to uphold the ruling against Aldo and require police to provide proof that the dog is able to do its job correctly. “There is no canine exception to the totality of the circumstances test for probable cause to conduct a warrantless search,” Gifford said. “If that is true, as it must be, any fact that bears on a dog’s reliability as a detector of the presence of drugs comes within the purview of the courts.”

Lawyer Gregory Garre, who represented the state of Florida in both cases, said they shouldn’t have to prove what kind of training and classes Aldo had, “the same way that when an officer provides evidence for a search warrant, we don’t demand the training of the officer, what schools he went to or what specific courses he had in probable cause.”

In Franky’s case, Garre argued that since it wouldn’t be illegal for a police officer to sniff for marijuana outside a door, it shouldn’t be illegal for a dog like Franky to do the same thing.

If that’s true, said Justice Ruth Bader Ginsburg, then police could just walk down a street with drug-sniffing dogs in “a neighborhood that’s known to be a drug-dealing neighborhood, just go down the street, have the dog sniff in front of every door, or go into an apartment building? I gather that that is your position.”

“Your Honor, they could do that,” Garre said.

But if someone invented a machine called the “smell-o-matic” that could do the same thing as Franky, police would not be able to use it outside of doors without a warrant, Justice Elena Kagan said.

Police aren’t allowed to use technology to see inside a person’s closed-up home without a warrant, argued Howard K. Blumberg, the lawyer for defendant Joelis Jardines. And the use of Franky outside the house “I would submit that would basically be the same thing as a police officer walking up and down the street with a thermal imager that’s turned on,” Blumberg said.

Justice Anthony Kennedy, who is often the deciding vote when the court is closely divided in a case, came down hard on both sides in Franky’s case. He told Garre, the attorney for Florida, that he didn’t agree with his argument that people with contraband inside their home don’t have an expectation of privacy. “Don’t ask me to write an opinion and say, Oh, we’re dealing with contraband here, so we don’t need to worry about expectation of privacy,” Kennedy said.

But Kennedy also told defense lawyer Blumberg that he won’t agree with his theory that it should always be considered a search when police try to find out what people are trying to keep secret.

To say “our decisions establish that police action which reveals any detail an individual seeks to keep private is a search: that is just a sweeping proposition that in my view, at least, cannot be accepted in this case. I think it’s just too sweeping and wrong,” Kennedy said.

“I would add a few words to the end of that statement: Anything that an individual seeks to keep private in the home, and that’s the difference,” Blumberg replied.

One Australian study found a dog only correctly identified drugs 12 percent of the time, Sotomayor said. “I’m deeply troubled by a dog that alerts only 12 percent of the time,” she said.

Garre argued that the numbers in that study could be read differently to raise that number as high as 70 percent, counting instances in which — even though drugs weren’t found — the person that the dog alerted to had used or been in proximity of drugs before the dog’s alert.

The justices will rule in the cases sometime next year.