"It is likely that these attacks were planned in advance," Gediminas Vysniauskis, deputy head of the tax inspectorate said in a statement Monday.
The tax office took its service offline to prevent damage and blocked access from some servers. Access to online tax filing was disrupted for several hours over the weekend, but no data was lost.
"Addresses of servers, which generated the requests, were blocked immediately. ... The preliminary analysis shows the servers were located in Romania," the office said.
Cyber-attacks are often routed through different countries in an attempt to disguise their true origin.
In June, a number of Lithuanian official web sites were hacked into and defaced with Soviet signs and anti-Lithuanian slogans, after the country's parliament banned the use of Nazi and Soviet symbols.
The Internet assaults are similar to those suffered by neighboring Estonia, when its servers were hit in April and May last year.
Estonia's head of Internet security linked those attacks to a decision by Estonia to remove a bronze statue of a Red Army soldier from the center of the capital, Tallinn.
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