Lapsa, who has written numerous books exposing corruption in high places and who runs the whistleblowing Pietiek.com website, told the BNS newswire he was held at the Security Police's headquarters after entering the building with a dictaphone.
"I was told that I am detained because I had a recorder with me and that it was not allowed to take one into the Security Police building. I was given no further explanations, except that force will be used against me if I resist. It's something like a local Gestapo," the journalist said.
His original intention had been to deliver a deposition to DP chief Normunds Mezviets, Lapsa said.
Just days ago Lapsa's website posted a bizarre transcript of his previous visit to the DP office which ended when officers could not provide certification for the equipment they were using to scan him.
Representatives of the Security Police told BNS that Lapsa was detained because he "did not comply with police officers' lawful requirements".
Lapsa later tweeted that during his detention he could hear his colleague and fellow investigative journalist Agnese Margevica being questioned about the sale of Citadele bank in the next room.
The detention of Lapsa could not be more inopportunely timed.
On May 3, Riga will play host to the World Press Freedom awards 2015, part of three days of conferences and discussions dedicated to media freedom with keynote speeches from high-ranking officials including President Andris Berzins.
A prominent attendee at the awards will be Australian-Latvian Peter Greste, recently released after spending more than a year in custody in Egypt.
While celebrating the release of Greste and the award of the main UNESCO award for press freedom to Syria's Mazen Darwish - currently in custody - it will prove farcical indeed if Latvia's own journalists are being detained in shady circumsstances for their reporting efforts at the same time.