Two bombs blasts took place near the entrance hall of a residential building for employees of Russia's Federal Security Service in Moscow on Friday evening, but no one was injured, police said.
The blasts occurred on Milashenkov Street close to the Timiryazevskaya metro station at around 4.30pm Moscow Time (1330 GMT).
Each bomb was equivalent to 200 grams of TNT, a police official said.
On Wednesday, an explosion ripped through a bus stop outside a training school for the security service in southern Moscow, injuring no one.
A police official said one criminal group was probably responsible for both the two incidents.
Data gathered from probes into Wednesday's explosion and Friday's blasts suggest that the blasts were carried out by the same people, the official said.
Both explosions have been classified as hooliganism.