Petraeus and Allen wrote letters in September to the District of Columbia Superior Court in support of the twin sister, Natalie Khawam, as she sought to gain more visitation rights with her son, according to a review of the court file.
The letters deepen the mystery of how two Tampa socialites developed close access to top military officials and raise questions about the specific nature of those relationships.
It is unclear why Petraeus and Allen, two of the US military's biggest names, felt the need to formally interject in a nasty custody dispute of someone they characterized as a family friend.
The court files also provide a glimpse into the connections Khawam had built with Washington elite, including Massachusetts Senator John Kerry.
Defense officials said earlier on Tuesday that Allen is under investigation for alleged inappropriate communication with Khawam's sister, Jill Kelley. Kelley is a fixture in military circles in Tampa, where she was a volunteer social liaison at MacDill Air Force Base.
Officials said they were combing through thousands of pages of email and other communications between Allen and Kelley.
Allen has denied that the two had a sexual relationship, officials said on condition of anonymity.
It was Kelley's complaints about harassing emails from the woman with whom Petraeus had an affair, Paula Broadwell, that prompted an FBI investigation that ultimately disclosed Petraeus' involvement with Broadwell. Petraeus resigned from the CIA post on Friday.
Khawam for months has been fighting for greater access to her 4-year-old son with Grayson Wolfe, a former Bush administration official who directed Middle East initiatives and Iraqi reconstruction efforts at the Export-Import Bank.
Wolfe could not be immediately located for comment. His LinkedIn profile identifies him as a founding partner of Akkadian Private Ventures, but when a Reuters reporter went to a Washington address listed for the firm, it was occupied by an unrelated private residence.
Wolfe, whose divorce from Khawam was finalized earlier this year, was awarded sole custody of their son in 2011.
District of Columbia Superior Court Judge Neal Kravitz found that Khawam "has extreme personal deficits in the areas of honesty and integrity." He cited false domestic violence petitions and a lack of honesty in her dealings with her family and employers.
On June 21 of this year, Kravitz ordered Khawam to pay Wolfe $350,000 in attorneys fees and costs, but Khawam continued to push for more access to her son.
A lawyer for Khawam did not immediately respond to requests for comment.
In his September 20 letter to the court, Petraeus said he had known Khawam for three years while serving in Tampa, through the friendship he and his wife, Holly, have with Jill Kelley and her husband, Scott Kelley, a Tampa cancer surgeon.
Petraeus said he had observed Khawam with her son during that time, including when the Petraeuses h osted them for Christmas dinner.
"In each case, we have seen a very loving relationship - a mother working hard to provide her son enjoyable, educational, and developmental experiences," Petraeus said.
Allen in his September 22 letter said he and his wife, Kathy, came to know Khawam through social functions while stationed at U.S. Central Command in Tampa. "She is a dedicated mother, whose only focus is to provide the necessary support, love and care for her son," Allen wrote.
Khawam not only tapped her military connections in her custody battle but also invoked broader contacts as she sought greater access to her son.
In emails with her ex-husband, Khawam wrote that Senator Kerry asked about the son and whether he would be coming to Martha's Vineyard in summer 2012.
She also wrote that Rhode Island Senator Sheldon Whitehouse invited the son to a summer clambake in Rhode Island, and that the son "would greatly appreciate attending this family clambake."
Included in the court file is what appears to be a handwritten note from Whitehouse, writing, "I am excited to hear that you, Natalie and (the son) may be coming to the family clambake. That would be terrific!"
The Whitehouse note was addressed to Gerald Harrington. Harrington served as national vice chairman of finance for Kerry's 2004 presidential campaign, according to a biography posted on the website of Harrington's lobbying firm, Capitol City Group.
A spokeswoman for Kerry said Kerry was introduced to Khawam by Gerald Harrington, whom the spokeswoman described as Khawam's boyfriend.
Harrington and a spokesman for Whitehouse did not immediately respond to requests for comment.