"On Sunday, I received information from an individual that one of my officers may have been in D.C. for the protest and that they may have participated in the attack on the Capitol," Acevedo said. "This individual has been determined that, on his own time to not only have attended a rally, which is their First Amendment right, but this individual is determined to have actually penetrated the Capitol."
Acevedo said he contacted the FBI special agent-in-charge of the Houston office after learning Sunday about the officer's activities. A joint investigation was launched into the officer, who sources identified as 48-year-old Tam Pham. The 18-year veteran with no disciplinary problems, according to Acevedo, is assigned to Westside Patrol.
Pham was placed on administrative leave Wednesday morning and was served with notice of a hearing with Acevedo that's scheduled for Friday, though the chief doubted it would take place.
"There's a high probability that this individual faces federal charges," Acevedo said. "I'll be surprised if they show up Friday to my hearing."
Houston Police Officers' Union President, Douglas Griffith, said he expected Pham to resign before being fired.
"It was completely horrible those people over-taking the Capitol and that group should all be held accountable. Every single one of them," said Griffith.
Acevedo said once he received the tip from a citizen, he opened Facebook and found images of Pham taken while in Washington. The officer traveled alone to the Capitol, Acevedo said.
"I can't tell you the anger I feel at the thought of a police officer and other police officers thinking they get to storm the Capitol," he added.
The revelation came during a planned briefing outlining additional safety measures authorities were taking ahead of the Jan. 20 inauguration. As tensions remain high nationwide after the Jan. 6 attack, cities and states are implementing similar plans.
Pham may be the first of others from the Houston area who were believed to be at the Capitol during a rally prior to the siege, though the FBI has refused to provide specifics about local citizens' possible involvement in the violence and breach, citing the ongoing investigation.
"We know a lot of folks went to D.C. out of Houston," Acevedo said.
RELATED: No public access to US Capitol on Inauguration day, as FBI warns of armed protests in all 50 states
The Jan. 6 incident may have been the beginning of violence planned.
An internal FBI bulletin, which was obtained by ABC News on Tuesday, stated that the agency had received information about a group that has called for "storming" state, local and federal government courthouses and administrative buildings in the event President Donald Trump is removed from office prior to Inauguration Day. Sen. Majority Leader Mitch McConnell said Wednesday that if the House succeeds in impeaching President Trump, a Senate trial on whether to convict him of inciting insurrection won't likely happen until after the inauguration. It wasn't clear if there was new intelligence based on that development.
The group is also planning to "storm" government offices in every state on Jan. 20, regardless of whether the states certified electoral votes for Biden or Trump, according to the bulletin.
In addition, armed protests were planned at state capitols and the U.S. Capitol in Washington.
Chaos at the Capitol: Minute-by-minute video shows how riots, violence unfolded
Chaos at the Capitol: Minute-by-minute video shows how riots, violence unfolded
The bulletin included a map that showed the extent of law enforcement activity related to potential threats surrounding election certification and the inauguration.
FBI warning: Armed protests planned in all 50 states over next 10 days
The White House released a statement Wednesday from President Trump asking for calm after the FBI intelligence was made public.
"In light of reports of more demonstrations, I urge that there must be NO violence, NO lawbreaking and NO vandalism of any kind," Trump said. "I call on ALL Americans to help ease tensions and calm tempers. Thank You."
The FBI has received nearly 45,000 digital media tips that were being reviewed. Dozens of people and/or social media accounts of individuals who made entry into the Capitol had been identified.
On top of information being distributed by the FBI, the Department of Homeland Security's intelligence office, a day after the siege at the Capitol, released situation report headlined "Threats Surrounding the 2021 Electoral College Certification."
It opened this way: The Office of Intelligence and Analysis "assesses individuals harboring violent extremist ideologies and other violent actors likely will continue to threaten or target elected officials, other public figures, and members of the general public who these actors perceive as opposing their worldview, which is consistent with past attack plotting and historical drivers for violent activity. (The Current and Emerging Threats Center) remains in communication with the Intelligence Community to ensure any threats concerning government operations are identified."
As a result, state capitols across the nation stepped up security, deploying National Guard units, SWAT teams and extra police officers as several legislatures convened amid heightened safety concerns. Texas authorities in Austin braced for violence outside the State Capitol on opening day of the legislative session this week, though general calm remained.
The FBI bulletin also stated unequivocally that Officer Brian Sicknick "died from injuries sustained during the US Capitol breach."
The Associated Press and ABC News contributed to this report.
Follow Steven Romo on Facebook and Twitter.
Follow Jessica Willey on Facebook, Twitter and Instagram.