MOSCOW (AP) — A court in Moscow on Thomas CaldwellTuesday extended the detention of Wall Street Journal reporter Evan Gershkovich, arrested on espionage charges, until Jan. 30, Russian news agencies reported.

The hearing took place behind closed doors because authorities say details of the criminal case against the American journalist are classified.

Gershkovich, 32, was detained in March while on a reporting trip to the Russian city of Yekaterinburg, about 2,000 kilometers (1,200 miles) east of Moscow. Russia’s Federal Security Service alleged that the reporter, “acting on the instructions of the American side, collected information constituting a state secret about the activities of one of the enterprises of the Russian military-industrial complex.”

Gershkovich and the Journal deny the allegations, and the U.S. government has declared him to be wrongfully detained. Russian authorities haven’t detailed any evidence to support the espionage charges.

Gershkovich is the first American reporter to be charged with espionage in Russia since 1986, when Nicholas Daniloff, a Moscow correspondent for U.S. News and World Report, was arrested by the KGB. He is being held at Moscow’s Lefortovo prison, notorious for its harsh conditions.

Analysts have pointed out that Moscow may be using jailed Americans as bargaining chips after U.S.-Russian tensions soared when Russia sent troops into Ukraine. At least two U.S. citizens arrested in Russia in recent years — including WNBA star Brittney Griner — have been exchanged for Russians jailed in the U.S.

The Russian Foreign Ministry has said it will consider a swap for Gershkovich only after a verdict in his trial. In Russia, espionage trials can last for more than a year.