A hacker, or hacking group, published via Twitter what they said were records of 9,000 DHS employees.
According to technology news site Motherboard, the hacker has said he will soon share the personal information of 20,000 DoJ employees, including staff at the FBI.
The news site said it had verified small portions of the breach, but also noted that some of the details listed appeared to be incorrect or possibly outdated.
In a statement, the DHS told journalists: "We take these reports very seriously, however there is no indication at this time that there is any breach of sensitive or personally identifiable information."
The Department of Justice also downplayed the breach's significance.
The hacker is understood to have used simple human engineering to bypass one stage of the authorities' security systems.
Motherboard quoted the hacker, who explained: "So I called up, told them I was new and I didn't understand how to get past [the portal].
"They asked if I had a token code, I said no, they said that's fine - just use our one."
The security of government systems was put under scrutiny last year when it was discovered that data on more than five million people was stolen from the Office of Personnel Management.
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The seven-hour standoff started after police tried to execute a warrant for the arrest of Korryn Gaines.
Gaines, who was armed with a shotgun, was shot dead in her apartment while her 5-year-old son watched.
Police said her followers on Facebook encouraged her not to give in.
According to Facebook's policies, accounts can be suspended if law enforcement believe individuals are at risk of harm.
Police Chief Jim Johnson has said their decision to request a de-activation was based upon the fact Gaines had a gun and a child was present during her standoff with officers.
Gaines' social media accounts have been restored. But two videos are missing and are being withheld for violating Facebook's standards - which include a provision against inciting violence.
Videos posted on Gaines' Instagram appear to show her talking to her son during the standoff.
In one clip, Gaines questions her son about the situation unfolding outside, with a police officer visible through her open door.
"What [are the police] trying to do?" she asks the boy.
"They're trying to kill us,' he replies.
Officers claim Gaines threatened to shoot them if they entered her apartment and that she fired shots several times.
Gaines' mother, Rhonda Dormeus, told The Baltimore Sun her daughter ignored pleas to surrender.
Ms Dormeus believes she might have been able to help calm her and end the situation peacefully had she been allowed to speak with her.
Gaines had a history of mental health problems, including anxiety, her former doctor said in an interview.
"I do feel like they didn't want to hurt her," Ms Dormeus said, referring to the police. "But I don't feel like they exhausted all the means of negotiation."
The initial warrant for Gaines' arrest related to a traffic stop charge dating back to March.
US authorities have acknowledged a data breach affecting the Department of Justice and Department of Homeland Security - but downplayed its severity.
[NEXT_CONCEPT]
A woman killed in a police standoff outside Baltimore tried to live stream the siege but authorities successfully had her social media accounts deactivated.